FCST 8/3/09 MO/IL/IA

[Ryan Wichman]

I can't believe I am starting a thread in August....It is still early in the game for this set-up but the 00z GFS and to a greater extent the 12Z NAM shows a favorable set-up severe weather over the upper mississippi valley early next week.

The way is appears right now, there should be on-going convection over this area along a cold front trailing from a 1004 low. But ahead of the cold front moderate destabilization is forecasted to occur (greater than 2500j/kg) . In addition sfc-500 shear values in the 30-40kt range should be sufficient for decent storm structure. Helicity and mositure also look in place as well...like I said, a few days out. But I deemed it necessary for a thread.

(With all of this being said, it IS 2009. Which means the final results will be scattered rain showers and possibly a few lightning strikes.)

 

[Matthew Piechota]

well monday good be a pretty descent day looking at the new WRF.

500 flow is out of the WNW and more NW into MO between 40-50kts in IA/MO with 850s out of the SW near 30kts with the highest speeds being further southeast into extreme southeast IA. I would like to see a better s/w moving through but just not seeing that right now. that could end up being the problem with CI monday afternoon, we will have to see.

CAPE readings are through the roof with MLCAPE AOA 3500 and SBCAPE possibly near 5000 in southern IA/northern MO and dew points up near 70 if not higher. looks like the 0z WRF is pooling moisture in northern MO with readings up near 75. theta-e is is high as sky and the axis goes right into northern MO and I-35 corridor in IA.

this model run has the CAP breaking by 0z from I-35 in IA east to ottumwa. as with models. this area with change with each run so will have to watch if it stays consistent. one thing I noticed with this run is that the CAPE values and dew points are higher then previous runs.

shear is pretty good and with 500 flow out of the WNW and NW you dont need backed sfc winds, southerly sfc flow would be just fine. EHI values are highest in northern MO and helicity values are also pretty good in southern to southeast IA, northeast MO.

One thing the models have been consistent on is the ongoing convection moving across IA earlier in the day. that will have to be watched as well as the clearing if/when it takes place behind it and also where. this model has is moving through and the sun coming out and skyrocketing the CAPE values up and producing agressive convection in southern IA by 0z.

right now, I would play the east of creston, IA to south of DSM on I-35 to ottumwa triangle, maybe a shift south in the whole triangle depending on that ongoing convection and expect storms to move southeast when they go up.


Matt

 

[Ryan Wichman]

Both the 00z NAM and GFS break out precip by 00z Tuesday in nearly the same spot. The NAM is a bit further to the east then the GFS which seems to break the cap with the insane instability.

One difference in the models from when I posted last, helicity values have jumped by nearly 100 over areas that the NAM is breaking out precip and SBCAPE is greater than 4500 j/kg. While things will likely change by Monday everning, if this proved to by the case and a discrete cell could break out it would be a pretty healthy storm.

 

[Jonathan Whitehead]

Main forecast conerns involve degree of ongoing precip and cloud cover across the region, specifically across Iowa and N. MO during the DY2 period. Severe storms are a definite in the threat area, the question is the degree of severe storms.

Mid summer death ridge will be spreading EWRD across the Plains bringing much warmer mid level temps EWRD with it. Being mid summer, widespread cloud cover should not be a problem across the threat area as subsidence and high insolation should burn off/break up any low level stratus that exists in the morning hours making for a nice Cu field.

Models are in overall agreement in transporting rich gulf moisture/70dews/NWRD overnight and into the morning hours across much of the Mo River Valley. This will jack SB instability into 4000 range within 35 kt NWLY flow...more than adequate for tilting of storm updrafts.

As morning turns into afternoon, next issue will be the strength of the cap. With the EWRD progression of the summer high, believe western half of MO will have stronger capping, thus believe ongoing storms across W/NW IL will transition to severe and will see a zipper-like effect of initialy discrete supercells WSWRD across W MO and northeast KS. Forecast hodos arent particularly curved due to veering component of sfc winds, however, discreteness of storms together with high instability may produce isolated brief tornado.

Overall believe most everyone has pretty good handle on tomorrow. Would not be surprised to see this go MDT with new DY1.