Quincy Posted June 23, 2015 Share Posted June 23, 2015 This event has had some discussion mixed in a few threads on the GL/OV subforum, but may be more appropriate here. Low pressure is forecast to be moving east across the central Plains Wednesday evening with a warm front draped over Iowa or perhaps far northern Missouri. The models seem to like a fading MCS or lingering convection moving through during the first half of the day. This could push an outflow boundary/effective warm front somewhat further south or perhaps disrupt the atmosphere a bit. Nonetheless, virtually all models show strong to extreme instability developing in the warm sector with considerable turning of the wind fields in the lower levels in vicinity of the warm front. Although there doesn't appear to be any significant shortwave forcing, a tendency for modest height falls seems likely and forcing along the front and ahead of the low pressure system may be sufficient for robust thunderstorm development. Even the Euro (along with the NAM/GFS/RGEM) all develop vigorous convection over Iowa by late afternoon and early evening. The placement is a bit variable in the models, ranging from southwestern Iowa via Euro to eastern Iowa/northwestern Illinois via GFS/RGEM. The NAM is somewhere between. Forecast soundings are highly impressive, depending on which station you use for the given model output. I don't think that air-mass recovery is a major issue and in fact, the help of an outflow boundary could only locally enhance a tornado threat. We are in the time of the year where length of daytime heating is nearly maximized. Last night, northern Illinois had no problem recovering, even into the very late afternoon/early evening. Wednesday night winds in the upper levels should remain near or just above 50 knots, with a LLJ increasing to 30 to 40 knots, so kinematic support is there. Link to comment Share on other sites More sharing options...
Quincy Posted June 23, 2015 Author Share Posted June 23, 2015 1730z SPC Day 2 extends the ENH risk into northern IL, but I wonder if that's more for wind - even though they did mention a strong tornado threat in IA or IL. Also mentioned a possible upgrade to MDT risk. Link to comment Share on other sites More sharing options...
David Reimer Posted June 23, 2015 Share Posted June 23, 2015 Good discussion from NWS DSM for Wednesday .LONG TERM /WEDNESDAY THROUGH TUESDAY/...ISSUED AT 343 PM CDT TUE JUN 23 2015SEVERE WEATHER OUTBREAK AND HEAVY RAIN POTENTIAL ON WEDNESDAYINTO WEDNESDAY NIGHT WAS THE MAIN CONCERN OVER THE EXTENDEDPERIOD. LEANED TOWARD A NAM/ECMWF BLEND FOR POP/WX TRENDSWEDNESDAY INTO WEDNESDAY NIGHT AS GFS SEEMS TOO FAR NORTH WITHTHE WARM FRONT AND THUS CONVECTION.WEDNESDAY INTO WEDNESDAY NIGHT...ONGOING MCS TONIGHT ISANTICIPATED TO PERSIST THROUGH THE MORNING HOURS OVER FARSOUTHERN IOWA AND LOOKS TO STAY SOUTH OF HIGHWAY 34 TO THEMISSOURI BORDER. QUESTIONABLE HOW FAR NORTH THE WARM FRONT WILLSURGE ASSOCIATED WITH THE SURFACE LOW EJECTING OUT OF THE CENTRALROCKIES TOMORROW. THIS IS CERTAINLY DEPENDENT ON THE MCS LOCATIONOVERNIGHT TONIGHT...BUT THE NAM/ECMWF AND EVEN THE GFS HAVE ADECENT HANDLE ON CURRENT CONVECTIVE TRENDS BUT DIFFER IN SOLUTIONSBY WEDNESDAY MORNING. THE WARM FRONT SHOULD SURGE NORTH TONORTHEAST THROUGH THE DAY WEDNESDAY AND LOOKS TO SET UP BETWEENINTERSTATE 80 TO HIGHWAY 20 AND BE SLIGHTLY ORIENTED NORTHWEST TOSOUTHEAST BY THE LATE AFTERNOON. STRONG IMPULSE PUNCHES INTOWESTERN IOWA BY THE AFTERNOON AND TRANSITIONS EAST THROUGH THESTATE INTO THE EVENING HOURS WEDNESDAY. STRONG WAA...MOISTURETRANSPORT AND THETA-E ADVECTION INTO THE STATE BEHIND THE WARMFRONT ALLOWING FOR THE ATMOSPHERE TO QUICKLY DESTABILIZE. THESURFACE BASED CAPE INCREASES TO AT LEAST 2500 J/KG BUT POSSIBLYRISING OVER 4500 J/KG BY THE LATE AFTERNOON ALONG THE MO/IABORDER. WEAK CAP IN PLACE...BUT AFOREMENTIONED UPPER LEVEL VORTMAX WILL BE STRONG ENOUGH TO INITIATE STORMS. DEEP LAYER SHEARRANGES FROM 45 TO 55 KNOTS AND 0-1 KM SHEAR RANGES FROM 20-25KNOTS WITH GOOD VEERING THROUGH THE LOWER LEVELS LEADING TO ADECENT TORNADIC POTENTIAL...ESPECIALLY IF STORMS TIE INTO THESURFACE BOUNDARY TO ENHANCE ANY LOW LEVEL VORTICITY. LARGE HAILAND WIND ARE ALSO HIGHLY LIKELY WITH THESE STORMS. HENCE...HIGHCONFIDENCE IN SEVERE WEATHER POTENTIAL ACROSS A GOOD PORTION OFTHE FORECAST AREA TOMORROW AFTERNOON THROUGH THE EVENING. LOWERCONFIDENCE IN LOCATION OF THE SEVERE WEATHER AS DEPENDENT ON WARMFRONTAL SURGE AND THUS LEFT LIKELY POPS MENTIONED OVER MUCH OFCENTRAL SECTIONS OF THE FORECAST AREA FROM WEST TO EAST. Link to comment Share on other sites More sharing options...
Quincy Posted June 24, 2015 Author Share Posted June 24, 2015 Still somewhat unclear about specifics tomorrow. The 00z NAM isn't initializing too well with current convective trends, but it tends to show minimal convective activity Wednesday morning and continues to develop extreme instability over the southwestern half of Iowa. I peeked at the HopWRF ensembles. Two members echo the operational NAM, but one member shows an smallish MCV-type system moving through Iowa during the mid to late morning. The HRRR, FWIW, it already showing a convective mess allover the state by midday. While I am not sure that I buy it (the model tanked today and was way over-convecting across New England when nothing was going on), I don't think that scenario can be discounted either. Nonetheless, even with junky convection and/or an early day MCS, the low-end scenario would still favor moderate instability and ample shear for a few severe thunderstorms. I have to lean toward the HRRR given the way this season has panned out, both in terms of early day convection and for the NAM to struggle in the short-term with convective initiation. My guess is that SPC will probably maintain an ENH risk with the first day 1 outlook and not go with an upgrade. If for some reason the morning only features minimal convective coverage, then perhaps it could be a big day. At this point in the season, I almost have to take the NAM solution and laugh at it, but it shows a narrow window 00-02z Thursday with intense supercells across Iowa. Following that, the storms quickly merge to form a significant MCS that transverses northern Illinois/Indiana and makes it all the way to western Ohio before dissipating. I guess that discussion should be directed to the GL/OV subforum. Link to comment Share on other sites More sharing options...
Jonbo Posted June 25, 2015 Share Posted June 25, 2015 Severe potential was a little less than expected but rivers all over Des Moines area are approaching record levels. Link to comment Share on other sites More sharing options...
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