Very hot, humid air over southern lower MI is likely to contribute to some scattered thunderstorm development across the area this afternoon into the night. These storms will have the potential for heavy rainfall as they move east at 20-30mph, which could cause localized flooding. There is a slight chance of some storms becoming strong or severe, but that risk is most prevalent over the thumb region of the state; most of any storms that develop are expected to carry very little hail or damaging wind risk, and virtually no tornado potential.
Also keep in mind the heat advisory in effect today until 8pm tomorrow: drink plenty of water, do not leave children or pets in cars, and stay indoors as much as possible. (Heat advisory text from NWS Detroit)
It is impossible to predict specifically whether any storms will hit Ann Arbor. As those at the Townie Street Party saw Monday night, it’s possible for the skies around downtown to look very ominous with thunderstorms, but for us to receive only a few drops of rain, especially with storms which are nearly stationary. Storms this afternoon will be similarly very localized.
Looking forward, we can expect Friday to start out the same: hot and humid. Thunderstorms are expected Friday and Friday night as a cold front comes south into the area, bringing slightly cooler temperatures for us. Friday’s storms will be strong, with some severe potential (damaging winds and large hail will be a risk across the area), and they are expected to bring torrential rainfall. Saturday will be a bit cooler, with some risk of thunderstorms.
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI 431 AM EDT THU JUL 18 2013 MIZ047>049-053>055-060>063-068>070-075-076-082-083-190900- MIDLAND-BAY-HURON-SAGINAW-TUSCOLA-SANILAC-SHIAWASSEE-GENESEE-LAPEER- ST. CLAIR-LIVINGSTON-OAKLAND-MACOMB-WASHTENAW-WAYNE-LENAWEE-MONROE- 431 AM EDT THU JUL 18 2013 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTHEAST MICHIGAN. .DAY ONE...TODAY AND TONIGHT A HEAT ADVISORY IS IN EFFECT TODAY AND TONIGHT AS HEAT INDEX VALUES REACH OR EXCEED 100 DEGREES DURING THE DAY AND ONLY FALL INTO THE UPPER 70S TONIGHT. PLEASE SEE THE LATEST FORECASTS AND STATEMENTS FOR DETAILS. SCATTERED THUNDERSTORMS ARE EXPECTED TO DEVELOP TODAY WITH A FEW STORMS CONTINUING OVERNIGHT. A FEW STRONG TO SEVERE STORMS MAY DEVELOP...WITH THE BEST POTENTIAL FOR SEVERE WEATHER OVER THE THUMB. STORMS WILL ALSO BE CAPABLE OF VERY HEAVY RAINFALL AS THEY MOVE TO THE EAST AT 20 TO 30 MPH. .DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY A HEAT ADVISORY REMAINS IN EFFECT THROUGH FRIDAY. THE PROLONGED PERIOD OF HOT DAYTIME TEMPERATURES AND WARM AND MUGGY NIGHTS WILL INCREASE STRESS ON THE BODY. THUNDERSTORMS ARE LIKELY FRIDAY AND FRIDAY NIGHT AS A COLD FRONT DROPS SOUTH THROUGH THE AREA. THERE IS A CHANCE FOR SEVERE WEATHER AND TORRENTIAL RAINFALL WITH THESE STORMS. THE THREAT FOR THUNDERSTORMS WILL CONTINUE INTO SATURDAY...HOWEVER COVERAGE AND INTENSITY WILL BE LESS. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION WILL NOT BE NEEDED TODAY AND TONIGHT. $$
And, for reference, the heat advisory text:
URGENT - WEATHER MESSAGE NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI 353 AM EDT THU JUL 18 2013 ...HOT AND HUMID CONDITIONS PERSISTING THROUGH FRIDAY... .A STRONG HIGH PRESSURE SYSTEM IS RESULTING IN AN EXTENDED STRETCH OF HOT AND HUMID CONDITIONS. VERY HUMID CONDITIONS COMBINED WITH TEMPERATURES RISING INTO THE MID 90S WILL RESULT IN HEAT INDICIES AROUND 105 DEGREES THIS AFTERNOON. IN ADDITION...OVERNIGHT RELIEF FROM THE HEAT WILL BE MINIMAL AS HUMID CONDITIONS PERSIST AND LOWS HOLD IN THE UPPER 70S. SIMILAR CONDITIONS ARE EXPECTED TOMORROW BEFORE A COLD FRONT PROVIDES RELIEF FOR THE WEEKEND. MIZ047>049-053>055-060>063-068>070-075-076-082-083-182015- /O.CON.KDTX.HT.Y.0001.000000T0000Z-130720T0000Z/ MIDLAND-BAY-HURON-SAGINAW-TUSCOLA-SANILAC-SHIAWASSEE-GENESEE- LAPEER-ST. CLAIR-LIVINGSTON-OAKLAND-MACOMB-WASHTENAW-WAYNE- LENAWEE-MONROE- INCLUDING THE CITIES OF...MIDLAND...BAY CITY...BAD AXE... SAGINAW...CARO...SANDUSKY...OWOSSO...FLINT...LAPEER... PORT HURON...HOWELL...PONTIAC...WARREN...ANN ARBOR...DETROIT... ADRIAN...MONROE 353 AM EDT THU JUL 18 2013 ...HEAT ADVISORY REMAINS IN EFFECT UNTIL 8 PM EDT FRIDAY... HAZARDOUS WEATHER... * HIGH HUMIDITY AND MAX TEMPERATURES IN THE LOW TO MID 90S RESULTING IN HEAT INDICIES OF 95 TO 105 DEGREES THROUGH FRIDAY. * LITTLE OVERNIGHT RELIEF WITH HUMIDITY PERSISTING AND LOWS IN THE UPPER 70S. IMPACTS... * HEAT EXHAUSTION OR HEAT STROKE IS POSSIBLE IN EXTREME CASES WHERE NO PROTECTIVE ACTION IS TAKEN. * MUSCLE CRAMPS AND SUN BURNS ARE ALSO POSSIBLE PRECAUTIONARY/PREPAREDNESS ACTIONS... * A HEAT ADVISORY MEANS THAT A PERIOD OF HOT TEMPERATURES IS EXPECTED. THE COMBINATION OF HOT TEMPERATURES AND HIGH HUMIDITY WILL COMBINE TO CREATE A SITUATION IN WHICH HEAT ILLNESSES ARE POSSIBLE. DRINK PLENTY OF FLUIDS...STAY IN AN AIR-CONDITIONED ROOM...STAY OUT OF THE SUN...AND CHECK ON RELATIVES AND NEIGHBORS. CHILDREN AND PETS SHOULD NEVER BE LEFT UNATTENDED IN VEHICLES UNDER ANY CIRCUMSTANCES. * PREPARE...PLAN...AND STAY INFORMED. VISIT HTTP://GO.USA.GOV/RR8 && $$