Monday 5 May 2008 Outlook

It appears some storm chase potential may set up from NW KS (perhaps NE CO along the Palmer Divide, as well) and southward along the dryline. This setup is ahead of a cut-off low developing over Baja California and an upper level jet streak entering the area later in the day. Because of the moderate westerly flow aloft, a low-level jet at 850 mb out of the south setting up and backed surface winds, the vertical wind shear profiles (including storm relative helicity) will be very favorable for supercells and isolated tornadoes. With a surface low in SE CO, if resolved correctly, a dryline bulge may appear just south of GLD as well. Surface dewpoints will be in the low 60s. With steep lapse rates, mixed layer CAPEs ~1500 J/kg result ahead of the dryline, extending around the surface low into eastern CO. However, a capping inversion (EML with warm temps at 850-700 mb) may be advecting in later in the day, increasing CIN. Additionally, it is unclear whether earlier convection predicted in the NAM may stabilize the warm and moist sector of interest. Therefore, initiation will be conditional on destabilization in clearing areas and must be caused by a sufficiently strong forcing mechanism. Any midlevel disturbance may help, dry line punch, although I suspect the Palmer Divide in eastern CO may be another interesting location for storm initation. Thus, I suspect potential target zones may extend as far south as the northern TX panhandle, and more likely into western KS/eastern CO.

Keep tuned for Tuesday's severe weather outlook.

C.R.

2200z Monday April 7, 2008

This is incredible...look at the hook on this baby!!!!

-Howell

April 8, 2008 Forecast: Midwest/OH Valley and SWward

Both NAM and GFS are in rough agreement on the synoptic situation on Tuesday. The upper level trough will continue to move eastward on Tuesday and the developing surface low will move through IL by the evening as it heads towards the northeast. A fairly broad warm sector, which will of course bypass Wisconsin, will cover IL/SE MO/AR/TN/KY/IN/etc by convective hour. To the southwest of this low, a cold front will be sweeping east-southeastward during the day. While the forcing and unidirectional wind profiles along much of the cold front would seem to favor a squall line, impressive SRH exists in the warm sector on Tuesday, especially northward toward the warm front and low, which may impinge upon the southern Chicago area by evening. If adequate sunshine exists in the northern part of the threat area, an interesting tornadic supercell threat may form tomorrow afternoon/evening. Further south, squall line will be likely mode, but again, shear will be strong enough for supercells and moisture quality will be even greater. Tough forecast for a chaser - I will look at the forecast in more detail after tonight's 00 Z runs to get a feeling on whether a southern or northern target is preferred.

C.R. 2:40 pm

April 7, 2008 Forecast: OK/TX

Interesting setup is emerging in southern OK and northeastern TX this afternoon. Lee side troughing and a developing low pressure system forming over northeastern NM is setting the stage for a possible supercell event. A warm front currently extends across southern OK, with a dryline running south of CDS. Surface winds are southerly to backed in the warm sector. Rich Gulf moisture is beginning to creep into the region of interest as well. Here's the SPC RUC/observation-based mesoanalysis showing much of the situation at the surface.
Shear and instability are already sufficient for supercells, and even tornadoes, but these parameters will improve through the evening, especially as the low-level jet picks up this evening and the upper level trough moves in. 18 Z sounding, just north of the warm front shows an already impressive wind profile and nice lapse rates above 700 mb.

Click to enlarge:

Supercells are likely to form along the northward moving warm front, and have good odds at being tornadic there. Other supercells may form along the N-S dryline. A no-brainer for local storm chasers today.


C. R. 2:30 pm

Radar Image from April 3, 2008...

Here is a reflectivity image from LZK (little Rock) at 0136z. This is a mature supercell with a hook echo, inflow notch, and some intense rainfall. This tornado formed in a severe thunderstorm watch, but conditions were still potent enough for tornado formation. Here are some useful indicies I looked at where this tornado formed:

Surface dewpoints: 63-64 degrees
LCL's: 1000 meters
LFC: 1400-1600 meters
0-3km Capes: 50-100 J/kg
MUCAPES: 1500-2000 J/kg
Significant Tornado Potential (STP): 3-4
0-6km Shear: 60 knots
Effective Bulk Shear (EBS): 60-65 knots
1km EHI: 1
3km EHI: 1 (these values would have been higher if there was higher cape)
1km SRH: 250-350 M2S2
3km SRH: 300-400 M2S2
9-11 km storm relative winds: 35 knots (high precip/almost classic supercell winds)
1km shear was also favorable at 30-35 knots.

This supercell storm occurred as nocturnal cooling was beginning, but this goes to show that it doesnt take much for the atmosphere to work with what it has. Always look at the whole picture when trying to figure out whats going on. You dont have to have a tornado watch for tornadoes to form, as illustrated.

-Howell
April 4, 2008

April 3, 2008 @ 20Z

Lets get ready for some supercells...the latest visible image shows some good TCU along the dryline telling us the cap is weakening and storms arent far off.

-Howell

Interesting Statistics Graph on Tornado Trends...

Found this interesting graph on the number of tornadoes that have occurred, comparing during the the last three years, compared to this year so far. The comparison is amazing. The trend is way above normal and significantly above the ten year average line. Click on the graph to see it full size.

A.D.

Supercells in OK

Unfortunately I don't have time to really do a good discussion at this point, and my forecast for today didn't exactly pan out as ambitiously as I had predicted, but these beautiful supercells are currently ongoing in OK.

C.R.

30 March 2008 Update

Models consistently show an interesting setup Sunday evening, particularly over Oklahoma and southern/eastern Kansas. A substantial short-wave trough is moving into the Rocky Mountains and will amplify over the next two days. The timing of the shortwave's entrance on the plains is not going to be timed well with the maximum surface heating tomorrow. Rather, the system starts to really interact with Tornado Alley soon after dark, so tomorrow's severe weather event may begin toward the end of daylight and really become a significant event as the sun sets. In terms of specific details, here are some facts to be concerned with:

A dryline will exist around the western OK/eastern TX panhandle border, extending south of a quasi-stationary baroclinic boundary running from the OK panhandle toward east-northeast into Kansas City. NAM has a bulge in the dryline south of the boundary. Lee troughing also is resulting in enhanced surface flow and some backing of the winds in the warm sector. Winds are particularly backed along the quasi-stationary frontal boundary. Thus, there will be surface boundaries, along with daytime heating, that may allow convection to develop in several locations before the sun sets. Foci of convective initiation may include the dryline bulge just east of Childress, TX and the triple point near the stationary front and dryline intersection in NW OK.

Because of low-level Gulf moisture and warming temperatures, along with cooler temperatures aloft, which are just starting to advect in, the NAM model has CAPEs over 2.5 kJ / kg in the warm sector over OK/southeastern KS.




Enhanced instability will continue to exist as the evening wears on. However, very impressive kinematics move into the warm sector throughout the night. Thus, tornadic supercell conditions will start out very good on Sunday afternoon and end up being very impressive during the nighttime hours. The low-level jet (850 mb flow) should ramp up from about 35 kts at 00 UTC to 55 kts by 06 UTC. As a result of improving flow throughout the evening, 0-3 km SRH improves from >200 J/kg to >500 J/kg over much of the area of interest during the same time period. A significant tornado event will exist with all storms in this region. Long-lived and potentially destructive tornadoes will be highly favored for any supercell that can move along the baroclinic boundary in N central OK into area just around or south of Kansas City, KS. Severe convection should become more widespread in the moving warm sector throughout the night as we head into a potentially more widespread severe weather day on Monday. A great chase scenario, but it might require some nighttime chasing. Regardless, I wouldn't be surprised by daylight tornadoes for the first storms that erupt in the late afternoon/early evening.

C.R. 1:21 pm Saturday.

Monday's Weather

I agree with Mr. Rozoff...things could get rather interesting on Monday evening. The GFS and NAM/WRF models are in fairly good agreememnt with the evolution of the next system entering the picture. The NAM/WRF model takes the area of low pressure of 1006 MB into central Illinois, but the GFS is a little deeper with 998 MB and holds it further west into portions of northeast Missouri/southeast Iowa. The cold front should sweep from central Missouri southwestward into southeast Oklahoma by 00z Monday. The low levels will be primed with a 50-70 knot LLJ from southeast Oklahoma into southeastern Missouri, and MLCAPES of 2100-2400 JKG in a corridor from Dallas to Wichita. Storm relative helicities of 300-500 M2S2 will also aid in some good "twisting" and plenty of moisture should be present with 60 degree dewpoints as far north as western Illinois. I think the brunt of the severe weather should impede portions of southeast Oklahoma, northwest Arkansas, and southcentral/southeastern Missouri. Although atmospheric indicies will be less further north, severe weather cannot be ruled out into central/southern Illinois as well. We will have to pay close attention to any storms that fire along the warm front as this is often the location that storms get into higher helicity values, increasing the tornado potential. This will be fun to watch. I will take anything after the winter we have had up here!!

Chris Howell (MESO Vice President)

27 March 2008 Snowfall

This is kind of neat. This MODIS image was forwarded to me by a colleague at my work showing the snowfall streaks from yesterday's snow event. While the snowfall didn't accumulate as deeply as it could have, snow swaths were left fairly close to the model predicted region of maximum snowfall (see Iowa/Illinois/southern WI/Michigan). No doubt the moderate temperatures and sun angle prevented more snowfall from falling, but the location of the model predicted snowfall wasn't shabby. The streaks exemplify where banding structures in the precipitation occurred.

Click to enlarge:



So, the models did fairly well on predicting the overall characteristics of yesterday's storm. The low set up slightly further northeast, and supercell conditions did prevail. Snowfall happened in the right areas.

As for Sunday, Monday, and future severe weather outbreaks, I will return this evening to post more on these events.

30 March 2008

Latest model runs show a fairly substantial upper-level trough moving into the Rocky Mountain West this weekend, with a strong disturbance moving into the plains on Sunday during the day. The 00 UTC NAM 500 mb flow is shown below. Fairly strong midlevel flow is shown over the high plains.


The surface pattern shows lee troughing associated with the midlevel trough as well. The NAM may be overestimating the 65 degree dewpoints that appear in the warm sector over OK and eastern TX, and I've often found this kind of setup to be unreliable as the models try to move the system onto the plains too rapidly. If this forecast rings true though, a fairly substantial severe weather event may be setting up for Sunday. What is more likely is that this pattern will set up more likely severe weather days into early and mid week following this weekend. At this point, a pattern worth noting, but I'm not confident in calling Sunday a model verification success yet due to my experience with this kind of "promising" forecast.

C.R. 10:55 pm C.D.T.

27 March 2008 evening update

Currently a nice line of supercells have formed over eastern OK up through central MO, as seen in the radar grab below. Defined cloud streets are seen in the ground clutter around the radar site exemplifying the direction of the low-level shear. These supercells are producing severe hail, up to 1.75" hail so far reported.



Currently, 0-6 km vertical wind shear is in the range of 40 - 70 kts and effective SRH approaches 200 J/kg across the entire domain of convection, which is certainly sufficient for supercells. SBCAPE is around 500 - 1500 J/kG where convection is currently occurring, and the convection is forming right along the strong cold front. Because of the lower quality of the moisture, LCLs are slightly high (~1.5 km AGL), so tornado threat may not be substantial, especially if cold front undercuts supercells.

Update (6:25 pm CDT):

Our first tornado warning has popped up in OK on the very nice looking supercell in the radar imagery shown below. This event is looking much more interesting.




The Peanut Gallery (9:58 pm CDT):

Well, there have been at least 4 tornado reports this evening, one out of the cell pictured above in OK and 3 in central MO associated with the line of supercells. One 2.75" hail report occurred with a tornadic storm in Pulaski County, MO. Apparently, the supercells were able to keep optimally far ahead of the cold frontal forcing boundary to sufficiently keep from being undercut by cold and stable low-level air, thus they were free to undergo tornadogenesis. Using the proximity sounding from SGF, the Bunkers right moving supercell index estimated that the storm motion of the supercells would be toward the east-southeast (100 deg) at 29 kts. Given some projection of motion away from the front along its normal heading, the relative speeds were apparently somewhat comparable. The strong cap may have prevented too much convection from forming along the front. Obviously the strong wind shear aided supercell development. A very interesting case that dispelled some of skepticism from earlier today.

27 March 2008 morning update

Since I do not have a lot of time this morning, I thought I'd give a very brief and unprofessional outlook for today. So, both this morning's RUC and last night's 00 UTC 4.5 km WRF run do not provide much indication of any hope for a good storm chase in the Red River region today. RUC and NAM from this morning show the system really setting up further to the northeast from last night's run and show a cap that is nearly impossible to overcome over the previously mentioned target area. In other word's, it comes as no surprise that last night's model runs were once again different than what the following runs would show. Given that the best case scenarios are marginal and today's outlook is dismal, I'm apt to say today is a bust over southern OK. More large-scale, heavy-rain producing convective systems should form to the northeast of the original target area, as expected. This will add more water to areas that are flood prone. As for the snow over southern WI and northern IL, the forecast shows a mixture of rain and snow. It is not looking like a substantial winter storm.

C.R. 9:35 am.

27 March 2008

Forecasting for tomorrow has been exceptionally difficult because model forecasts have been anything except convergent on a single solution. Part of the reason for this rests in the progressive, relatively small amplitude nature of the upper level disturbance, which is embedded in predominantly zonal flow.


















Tomorrow's winter weather

Yesterday's GFS and NAM model runs had a fairly decent snowstorm setting up over MN/WI/MI for tomorrow with 6" or more snow accumulation over a fairly narrow zonal band north of the warm front and ahead of a surface low pressure system. This morning, the model runs showed very insignificant accumulations of much precipitation of liquid or frozen phase in this region. But once again, this evening, a fairly decent swath of precipitation appears in the 00 UTC NAM and GFS, and this time the band sits over northern IL. This NAM suggests over 0.5" precipitation over the Chicago area during the day tomorrow in a region of frontogenesis, and if this were in the form of snow, accumulations of over 5" are not out of the question. However, the temperatures will be around or above freezing near the surface, which will probably give northern IL a mixture of rain and snow. Given the disappearance and reappearance of this snowstorm in various model runs, this appears to be something of a concern. Now that the weather service cancelled all winter storm watches, will we see new snow advisory or winter storm warnings posted tomorrow somewhere in a zonal band over northern IL or possibly southern WI? Currently accumulating snowfall is occurring over eastern SD and southern MN. This band is headed toward our area.

NAM's 12-h accumulated liquid-equivalent precipitation by 00 UTC tomorrow evening is posted above.

Severe Weather

Yesterday's model runs also hinted at the possibility of some marginal to locally enhanced severe weather tomorrow. This morning, the model runs were showing a substantial capping inversion over the most favorable kinematic and thermodynamic region for supercells (i.e., OK). Thus, the severe weather outlook has been as uncertain as the resolved model details surrounding this system. While tomorrow's outlook still looks somewhat unentertaining in the latest model runs, there could be some severe opportunities that crop up in the warm sector. The image below shows the NAM's forecast position of the surface low, over the TX panhandle, with a surging cold front over northern OK and MO. Warm and moist low-level flow throughout the warm sector will provide energy for thunderstorms that may form along the frontal boundary, all the way through the OH River Valley. The dryline south of the low pressure center would be the other focus for thunderstorms. Moderate to strong vertical wind shear should aid in storm organization, whether the storm mode is squall line (more probable along the cold front) or supercells, which would be likely if storms can overcome the strong EML-induced capping inversion, which will require ample insolation and most preferably forcing from surface boundaries.

This scenario is also portrayed in the latest GFS, but given the lack of temporal consistency in model runs, confidence is still low in the exact details of the surface flow and pressures tomorrow. While convection is more likely northeast of the surface low in the TX panhandle, my ideal position would sit somewhere near the Red River Valley in OK, just east of the forecasted dryline bulge (seen in the 00 Z 28 March NAM dewpoint plot):
There is high bust potential due to the stout cap, but if this cap breaks in my ideal target zone, the 2500 J/kg + MLCAPE and marginal to decent storm relative helicity (> 100 J/kg 0-1 km srh) may provide for isolated tornado development, and at least a photogenic supercell or two, that is, if the storms can form before the sun sets. The other drawbacks to this event are the SWrly flow just above the surface advecting in drier air, which could negatively affect the moisture available to storms. Other severe threats, although perhaps marginal, should include hail and and wind along the cold front, from central OK toward the northeast into the Ohio River Valley. Embedded supercells may enhance severe weather in these regions, but the threat again is conditional upon the strong cap breaking, which will be more likely as one heads toward the northeast into less favorable thermodynamic and kinematic conditions. Overall, tomorrow is still a very marginal day severe weatherwise, but the threat is there for some severe weather. Won't be surprised to see SPC post a slight risk over this region if the model runs don't flake out again on us. But this is a very risky storm chase situation, with a nice surprise if that relatively small probability supercell can overcome the potential negatives.




C.R. 11 pm

Welcome

This site serves as a central hub for forecast discussions for MESO. MESO members will occasionally update this site with forecasts for interesting storm systems and weather events. This site will be especially active during severe weather episodes, particularly during MESO storm chases. Please check here often.

For MESO forecasters, please login to this site to leave a forecast. Forecast discussions should be entitled by a date. In case of multiple forecast discussions for a given day, please also post the time of your forecast.