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Weather Story NWS Chicago, IL |
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Low pressure in the northern plains will move southeast towards the Midwest on Friday. Clouds will be on the increase through the day and there will be a chance for showers and thunderstorms across northern Illinois and northwest Indiana beginning Friday afternoon and continuing into Saturday morning.
000 FXUS63 KLOT 300342 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 1042 PM CDT THU JUL 29 2010 .DISCUSSION... 827 PM CDT IR IMAGERY SHOWING A NARROW CHANNEL OF CIRRUS SLIDING SE ALONG THE LEE-SIDE OF THE MID-LVL RIDGE AXIS IOWA...NORTHERN IL...SOUTHERN WI. ADDITIONALLY A COMPLEX OF TSRA DEVELOPING OVER NORTHCENTRAL NEBRASKA/SOUTHERN SOUTH DAKOTA WITH DECENT 850MB MOISTURE FLUX CONVERGENCE AND A BETTER SHEARED ENVIRONMENT. FOR THE GREAT LAKES ANTI- CYCLONIC FLOW IN THE LLVLS WILL MAINTAIN A DRY ATMOSPHERE OVERNIGHT. LATEST SREF/NAM12 GUIDANCE HINTS THAT PRECIP ARRIVAL MAY BE DELAYED UNTIL CLOSER TO 00Z SAT FOR NORTHERN IL. OVERNIGHT LOWS WILL COOL INTO THE 60S. WINDS WILL REMAIN LGT AND VRB THRU DAYBREAK FRI. BEACHLER //PREV DISCUSSION... 330 PM CDT THERE WILL BE A CHANCE OF THUNDERSTORMS FRIDAY. WARMER WEATHER WILL RETURN MONDAY. TONIGHT...THE SHOWERS AND THUNDERSTORMS OVER SOUTH DAKOTA AND IOWA WILL CONTINUE TO MOVE EAST. THIS AREA OF CONVECTION IS SEEN AS A VORTICITY MAXIMUM IN THE FORECAST MODELS. THE LOCAL WRF MODEL MOVES THIS VORTICITY MAXIMUM INTO SOUTHWEST WISCONSIN BY 06 UTC FRIDAY. THEN THE MODEL MOVES THE VORTICITY MAXIMUM INTO ILLINOIS BY 12 UTC. WE WILL PUT A CHANCE OF THUNDERSTORMS IN THE FORECAST FOR NORTHWEST ILLINOIS AND NORTH CENTRAL ILLINOIS LATE TONIGHT AND EARLY FRIDAY MORNING. MEANWHILE A SURFACE HIGH WILL MOVE ACROSS NORTHERN ILLINOIS AND NORTHWEST INDIANA TONIGHT AND EARLY FRIDAY. THIS ANTICYCLONIC SURFACE FLOW MAY INHIBIT SURFACE CONVERGENCE FOR THUNDERSTORMS. SO THAT IS ANOTHER REASON WE WILL LEAVE A CHANCE OF THUNDERSTORMS IN THE FORECAST TONIGHT. FRIDAY...THE HIGH WILL MOVE EAST. A LOW WILL DEVELOP OVER IOWA LATE FRIDAY. THIS LOW WILL MOVE FROM IOWA TO CENTRAL ILLINOIS. THIS WILL KEEP A CHANCE OF THUNDERSTORMS OVER NORTHERN ILLINOIS FRIDAY NIGHT. WE USED THE GFS MODEL BECAUSE OF THE TIMING OF THE 500 MB HEIGHT PATTERN WAS CLOSE TO THE MOTION OF THE CLOUDS ON THE SATELLITE LOOPS. SATURDAY AND SUNDAY...A HIGH WILL MOVE INTO NORTHERN ILLINOIS. THE HIGH WILL SPREAD EAST INTO MICHIGAN AND ONTARIO BY SUNDAY EVENING. MONDAY THROUGH WEDNESDAY...ANOTHER FRONT IS FORECAST TO MOVE ACROSS ILLINOIS AND NORTHWEST INDIANA BY TUESDAY. WE WILL KEEP A CHANCE OF THUNDERSTORMS IN THE FORECAST FOR MONDAY NIGHT AND TUESDAY. WE EXPECT WEDNESDAY WILL BE DRY WITH HIGH TEMPERATURES IN THE MIDDLE 80S. && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 04Z... NO SGFNT OPERATIONAL CONCERNS FOR THE REMAINDER OF THE EVENING AND OVERNIGHT HOURS WITH LGT/VRBL WINDS AND MID/HIGH CLOUDS. MAIN UPCOMING CONCERNS INVOLVE WIND DIRECTION FORECAST FOR FRIDAY AND CHANCES OF SHOWERS OR THUNDERSTORMS FRIDAY EVENING INTO FRIDAY NIGHT. KREIN //DISCUSSION...UPDATED 00Z... WITH LOW LEVEL RELATIVELY DRIER AIR INFILTRATING AT THE SURFACE AND HIGH PRESSURE MOVING ACROSS THE LOCAL AREA TONIGHT...THE ONLY CLOUD COVER OF CONSEQUENCE TONIGHT WILL BE INCREASING MID AND HIGH CLOUD SPREADING IN ADVANCE OF THE CONVECTIVE CLUSTER OVER IOWA. AS THE CENTER OF THE HIGH SETTLES OVER SOUTHERN WISCONSIN AND SOUTHERN LAKE MICHIGAN...WINDS WILL BECOME LGT/VRBL OR CALM AFTER SUNSET AS DAYTIME MIXING SHUTS OFF AND THE PRESSURE GRADIENT BECOMES ALMOST NON-EXISTENT. INCREASING CLOUD COVER AND LOW SFC DEWPOINTS WILL NOT BE CONDUCIVE TO THE DEVELOPMENT OF ANY VSBY RESTRICTIONS DESPITE THE CALM WINDS. ON FRIDAY...THE SFC HIGH WILL MOVE OFF TO THE EAST AND LOW LEVEL WARM ADVECTION WILL INCREASE IN ADVANCE OF A WEAK UPPER DISTURBANCE APPROACHING FROM THE NORTHWEST. LOW LEVEL MOISTURE AVAILABILITY REMAINS A SIGNIFICANT CONCERN FOR THE POTENTIAL FOR THE DEVELOPMENT OF SHOWERS AND THUNDERSTORMS. WITH THE HIGH SLOWLY PUSHING OFF TO THE EAST AND A DEVELOPING FRONTAL TROF MOVING ACROSS THE CNTRL PLAINS...SFC WINDS WILL INCREASINGLY BECOME MORE SOUTHERLY...WHICH WOULD BE CONDUCIVE TO A RETURN FLOW OF MOISTURE. HOWEVER..SOME OF THE MODEL GUIDANCE IS SUGGESTING THAT LOW LEVEL FLOW COULD BE MORE SOUTHEASTERLY DURING THE AFTERNOON AND INTO THE EVENING...WHICH WOULD DELAY THE RETURN FLOW OF MOISTURE UNTIL LATER IN THE EVENING. SO...AT THIS POINT...EXPECT THAT THE INCREASING SHRA/TSRA CHANCES WILL NOT OCCUR UNTIL THE EARLY EVENING HOURS AND THEN WITH BETTER PCPN CHANCES LATER IN THE EVENING AND OVERNIGHT. KREIN //ORD AND MDW CONFIDENCE...UPDATED 04Z... HIGH CONFIDENCE IN VFR CONDITIONS THROUGH THE PERIOD. HIGH CONFIDENCE IN WIND FORECAST THROUGH EARLY FRIDAY MORNING. MEDIUM CONFIDENCE WITH WIND DIRECTIONS FRIDAY AFTERNOON/EVENING. HIGH CONFIDENCE IN TERMINALS REMAINING DRY THROUGH 00Z SATURDAY. LOW CONFIDENCE ON THE CHANCES OF THUNDER/PCPN LATE FRIDAY AFTERNOON/EARLY EVENING. KREIN //OUTLOOK FOR ORD/MDW FOR 06Z SATURDAY-12Z WEDNESDAY...UPDATED 18Z... FRIDAY NIGHT...CHANCE OF TSRA. OTHERWISE VFR. SATURDAY...SLIGHT CHANCE OF TSRA. OTHERWISE VFR. SUNDAY...VFR. MONDAY...CHANCE OF TSRA. OTHERWISE VFR. TUESDAY...VFR. WEDNESDAY...CHANCE OF TSRA. OTHERWISE VFR. MARSILI && .MARINE... 241 PM CDT WEAK GRADIENT SHOULD PERSIST ACROSS LAKE MICHIGAN THROUGH MUCH OF THE FORECAST PERIOD WITH THE STRONGEST WINDS OF THE PERIOD POSSIBLE DURING EARLY PORTIONS OF NEXT WEEK. SFC HIGH PRESSURE WILL CONTINUE TO BUILD INTO THE WESTERN GREAT LAKES REGION TONIGHT ALLOWING FOR A WEAK SURFACE PRESSURE GRADIENT TO BE MAINTAINED. BY LATE TONIGHT INTO FRIDAY MORNING...SHOULD START TO SEE A SOUTHEAST COMPONENT PREVAIL AS THE SFC HIGH MOVES OFF TO THE EAST. A WEAK SFC LOW ACROSS THE CENTRAL PLAINS WILL DRIFT EASTWARD OVER NEXT 36 HOURS AND TEND TO WEAKEN ACROSS THE MID MS RIVER VALLEY ALLOWING FOR ANOTHER HIGH PRESSURE AREA TO MOVE IN FOR SATURDAY. A STRONGER LOW PRESSURE AREA WILL TRACK ACROSS SOUTHERN CANADA MONDAY PERHAPS SETTING UP A STRONGER SOUTHERLY FLOW. STILL SOME UNCERTAINTY AS TO THE DEPTH OF THIS LOW...BUT COULD SEE SOME SOUTH GUSTS APPROACHING 20 KNOTS FOR EARLY PARTS OF NEXT WEEK. MARSILI && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$