Current thinking on heavy rain and severe potential tonight

Radar Update: While the heavy rain threat will continue to press east, it doesn't look as intense as it did earlier this evening in NW Illinois.  However, heavy rain will continue through the overnight with rainfall rates still pushing close to an inch per hour in localized spots.  Remain alert on the commute Friday morning.


10:50:08 PM) nwsbot: Local Storm Report by NWS DVN: Galena [Jo Daviess Co, IL] law enforcement reports FLASH FLOOD at 10:40 PM CDT -- several roads flooded in galena and being barricaded.





URGENT UPDATE:  Flash Flood Warning has been issued for Stephenson, Jo Daviess and Carroll counties until 4:30am.  More than two inches of rain has fallen in NW IL and flash flooding is expected to develop soon!  This is developing into a serious situation in NW IL

Also - Flash Flood Warning now issued for Green County in southern Wisconsin.

10:10pm Update: 24 hour rainfall totals so far.  This does include the rain that fell this morning.
 









9:45pm Update: Growing more concerned with this rain band setting up across far NW Illinois in Jo Daviess and NW Stephenson counties.  Flooding could become an issue





9:15pm Update: Heavy rain moving into Jo Daviess County with rainfall rates up to an inch per hour.  Rain will continue for the next couple of hours.  Some of the rainfall totals coming out of the Cedar Rapids, IA area have been close to two inches of rain within an hour!

 





Mesoscale discussion from the Chicago National Weather Service concerning heavy rain threat tonight:

STORMS OVER WESTERN ILLINOIS CONTINUE TO BE A MUTTLED MESS AND
CONVECTION HAS FAILED TO BECOME WELL ORGANIZED. GIVEN THE LOSS OF
DAYTIME HEATING...NO ALREADY WELL ESTABLISHED DISCRETE CELLS
GOING...AS WELL AS A OVERALL SLIGHT WEAKENING TREND IN THE STORMS
THINK THE SEVERE THREAT IS QUICKLY DIMINISHING. AFTER COORDINATION
WITH DVN...ILX...AND SPC HAVE CANCELLED THE WATCH EARLY.

FOCUS WILL NOW TURN TOWARD THE HEAVY RAIN THREAT TONIGHT. AS
ALLUDED TO IN PREVIOUS DISCUSSIONS...STRENGTHENING LOW LEVEL JET
IMPINGING ON THE QUASI-STATIONARY FRONT WITH VERY HIGH PWATS
POINTS TOWARD A HEAVY RAINFALL AND FLASH FLOODING THREAT
OVERNIGHT. REGIONAL RADAR MOSAIC SHOWS POTENTIAL MCV DEVELOPING
WITH CONVECTION OVER SOUTH CENTRAL IOWA...CONCEPTUALLY COULD SEE
WARM ADVECTION WING OF VERY HEAVY RAIN PRODUCING CONVECTION
LIGHTING UP OVER THE NEXT COUPLE HOURS. UNTIL CONVECTION DEVELOPS
AND BEGINS TO LINE UP HARD TO PINPOINT THE LOCATION WITH THE
GREATEST THREAT...BUT NORTHERN ILLINOIS WOULD CERTAINLY SEEM TO
STAND THE HIGHEST CHANCES. GIVEN THE VERY HIGH FREEZING LEVELS AND
HIGH PWATS...CONVECTION WILL LIKELY BE EXTREMELY EFFICIENT RAIN
PRODUCERS TONIGHT

8:10pm Update:  Training thunderstorms are already occurring in Northeast Iowa and could affect far Northwest Jo Daviess County through 9pm.  Doppler radar rainfall estimates of nearly two inches of rain have fallen since 7pm.


I think going into the evening and overnight our greatest threat will be the potential for heavy rain, more than the severe threat.

A Flash Flood Watch remains in effect for northern Illinois, while a Flood Watch has been issued for southern Wisconsin.  The Tornado Watch that was issued earlier for Whiteside and Lee counties has been canceled early.

The stronger storms this evening have been closer tied to where the warm front is in west-central Illinois and just south of the Sterling/Dixon area.  There, a few storms did have signs of rotation with one tornado being reported in Reynolds, IL (Mercer County) earlier this afternoon.  As we go further into the evening and overnight. the severe weather threat will diminish a little, but not completely. 

The low level jet, which is an area of stronger winds in the atmosphere located several thousand feet above the surface, is forecast to increase from the southwest late tonight.  This will occur as low pressure moves northeast from the Kansas/Missouri area.  The significance of the low level jet is that it can keep the building of thunderstorms going into the night.  Many times, as the low level jet interacts with warm fronts or stationary fronts, the risk for training thunderstorms increases.  Training thunderstorms are storms that move over the same locations over and over again.  It can also help sustain enough wind shear (changing of wind direction/speed with height) in the atmosphere to keep the severe weather threat as well.  However, I don't see this being a big issue for us going into the overnight.  Of greater concern will be the potential for heavy rain and possible flooding.

Right now, the warm front remains south of the immediate area.  Any storms that fire up closer to the front could contain a few stronger wind gusts.  This is also in an area that has dealt with significant rainfall over the past couple of days.  Heavy rain falling on an already saturated ground will only enhance the risk for flooding. 

Closer to home, we may be able to handle a little more rain, but not if it comes down in a continuous fashion for several hours.  Many of our short term models are indicating the heaviest rain to come down after Midnight and last through about 4am/5am Friday morning.  By 7am/8am Friday morning, most of the heavy rain should be done with.  Forecast models are indicating between one and three inches of rain falling through the night, which seems pretty much on track.

I will be in the weather center as long as the weather threat remains.  Any updates going into the night will be found here.  So, make sure you bookmark this page!