---------- Forwarded message ----------
From: Philip C. Ordway <pcordway@gmail.com>
Date: Wed, Jun 22, 2016 at 4:50 PM
Subject: tornado watch likely
To: "go@blogger.com" <go@blogger.com>
SPC has put the entire region at 95% likelihood of a tornado watch within the hour. I would expect it to be issued any moment. Excerpts of the MSD are below. There is certainly plenty of spin in the air (see below) so any severe storms that do develop could easily produce a tornado.
From: Philip C. Ordway <pcordway@gmail.com>
Date: Wed, Jun 22, 2016 at 4:50 PM
Subject: tornado watch likely
To: "go@blogger.com" <go@blogger.com>
[Note: the current storms in the area are producing some lightning and brief heavy rain, but they are NOT severe and NOT the potentially severe storms that could arrive this evening.]
The bad news, beyond the helicity, is the rapid destabilization of the atmosphere. As expected, warm moist air is flooding into the lower part of the atmosphere and it will sound ascend into newly formed storms. The CAPE (a measure of convective potential available energy) has jumped to >2,000 joules per kg -- well into severe territory.
The good news is that the heart of the severe and tornado threat remains just to the west of Chicago. Let's hope that any destructive storms that do develop stay over undeveloped areas. The exact frontal boundary will determine the specifics, which are impossible to pinpoint inside of 25-50 miles.
The most likely timing for severe storms in the Chicago area is 7-10pm. A couple of models are putting one or two discrete supercells in the area around 8:00, but such precision is tricky at best.
Not that it needs to be said, but the airports are obviously going to be a mess.
And again, the current NWS/SPC probabilities are 10% for a tornado within 25 miles of any point in the Chicago area, 45% chance of damaging winds, and a 30% chance of >1" hail. Those are meaningful chances -- and I would take the over on all three -- but they are far from certainties. This is also your daily reminder than if a storm produces severe weather in Schaumburg or Oak Park but not directly over your condo in River North, nobody cares and you're missing the point -- a severe storm still occurred in the forecast area.
MESOSCALE DISCUSSION 1020 NWS STORM PREDICTION CENTER NORMAN OK 0338 PM CDT WED JUN 22 2016 AREAS AFFECTED...ERN IA...FAR SRN WI...NRN INTO CNTRL IL...NW IND CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY VALID 222038Z - 222215Z PROBABILITY OF WATCH ISSUANCE...95 PERCENT SUMMARY...SCATTERED THUNDERSTORMS ARE EXPECTED TO DEVELOP BY 23Z. THESE STORMS WILL QUICKLY BECOME SEVERE WITH TORNADOES...DAMAGING WINDS AND LARGE HAIL POSSIBLE INTO THE EVENING HOURS. A TORNADO WATCH WILL LIKELY BE NEEDED WITHIN THE NEXT HOUR FOR PORTIONS OF THE MCD AREA. DISCUSSION...CONVECTION DURING MOST OF THE MORNING HOURS AND RELATED CLOUD COVER ACROSS IA/WI/IL/IND HAS RESULTED IN MAINTENANCE OF STRONG CAPPING /AS EVIDENT IN 19Z REGIONAL RAOBS/. HOWEVER...RECENT CLEARING IN THE LAST 1-2 HOURS HAS ALLOWED FOR STRONG SURFACE HEATING WITH TEMPERATURES CLIMBING FROM THE UPPER 60S F INTO THE MID 70S TO LOW 80S F AS FAR NORTH AS THE IL/WI BORDER. ADDITIONAL HEATING AND MOISTENING SHOULD CONTINUE TO ERODE THE CAP. AS DEEP LAYER FORCING FOR ASCENT INCREASES WITH THE APPROACH OF THE UPPER SHORTWAVE IMPULSE...STORMS ARE EXPECTED TO DEVELOP ACROSS ERN IA/SW WI/FAR NW IL BY 23Z. ONCE STORMS DEVELOP...THEY ARE EXPECTED TO TRACK E-SE ALONG A STRONG INSTABILITY GRADIENT IN THE VICINITY OF A WARM FRONT DRAPED ACROSS NRN IL INTO CENTRAL IND. STEEP MIDLEVEL LAPSE RATES NEAR 7.5 C/KM PER DVN AND ILX 19Z RAOBS AND 0-6 KM SHEAR GREATER THAN 50 KT WILL SUPPORT ROTATING UPDRAFTS AND LARGE HAIL. FURTHERMORE...RICH BOUNDARY LAYER MOISTURE WITH MIXING RATIOS NEAR 14 G/KG AND BACKED LOW-LEVEL WINDS EVIDENT IN 19Z RAOBS AND...MORE RECENTLY...INDICATED BY REGIONAL VWP/S...WILL SUPPORT A FEW TORNADOES /POTENTIALLY ONE OR TWO SIG TORS/. STORMS ARE EXPECTED TO GROW UPSCALE INTO A SOUTHEASTWARD PROPAGATING MCS WITH A COINCIDENT INCREASE IN DAMAGING WIND THREAT INTO INDIANA...AND LATER TONIGHT THE OHIO VALLEY.
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