000
FXUS63 KMPX 081111
AFDMPX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
611 AM CDT MON SEP 8 2008
.AVIATION... /12Z TAF ISSUANCE/
EXTENSIVE CS COVERS THE TAF SITES EARLY MORNING AS AN UPPER TROF
APPROACHES FROM THE CENTRAL DAKOTAS. MID LEVEL COLD TROF ALONG THE
DAKOTAS/MN BORDER WILL SPREAD ACROSS THE FA TODAY WITH STEEP
LAPSE RATES DURING THE AFTERNOON. NAM BUFKIT SHOWING LAPSE RATES
OF BETTER THAN 8 DEG C/KM IN THE 900-700MB LAYER THIS AFTERNOON
ALONG WITH 150 J/KG OF CAPE. EXPECT SCT CU TO BECOME BKN BY LATE
MORNING OR EARLY AFTERNOON WITH CEILINGS AROUND 5000 FEET. NSSL
4KM WRF SHOWING SOME SHOWERS DEVELOPING BY 18Z AND LASTING UNTIL
ABOUT 01Z. PV ANOMALY MOVING THROUGH AS WELL TODAY WITH STRONG
500-300MB Q-VECTOR CONVERGENCE OVERHEAD EARLY THIS AFTERNOON. AT
THIS POINT...INSERTED VCSH FOR THE TAF SITES THIS AFTERNOON. KEPT
BKN AC IN FOR EARLY TONIGHT AS AC SHIELD CURRENTLY OVER THE
WESTERN DAKOTAS SPREADS EAST AND SLOWLY DISSIPATES.
&&
.DISCUSSION... /ISSUED 326 AM CDT MON SEP 8 2008/
BROAD UPPER TROUGHING CONTINUES TO BE IN PLACE OVER CENTRAL NORTH
AMERICA THIS MORNING WITH SEVERAL EMBEDDED TROUGHS RUNNING THROUGH
IT. UPPER RIDGING IS IN PLACE UPSTREAM TO THE NORTHWEST OVER WESTERN
BRITISH COLUMBIA EXTENDING SOUTHWEST INTO THE PACIFIC OFFSHORE OF THE
U.S. PAC NORTHWEST. A DEEP UPPER LOW CONTINUES TO CIRCULATE OVER
SOUTHERN ALASKA AND THE ALEUTIANS WITH WATER VAPOR IMAGERY SHOWING
A POTENT TROUGH AXIS ROUNDING ITS BASE. THE EVOLUTION OF THIS TROUGH
WILL BE AFFECTING OUR WEATHER BY MID WEEK. AT THE SFC HIGH PRESSURE
ACROSS THE DAKOTAS WAS NOSING INTO MINNESOTA. LATE EVENING IR
SATELLITE IMAGERY SHOWED HIGH CIRRUS OVER MUCH OF FAR SOUTHERN
MINNESOTA AND WISCONSIN WITH MOSTLY CLEAR SKIES TO ITS NORTH...OR
ACROSS THE NORTHERN CWA WHERE TEMPERATURES HAVE DROPPED OFF SHARPLY
AS EXPECTED. HIGHER CLOUDS HAVE NOW MOVED OVER MOST OF THAT AREA.
WATER VAPOR IMAGERY SHOWS A TROUGH ENTERING THE WESTERN DAKOTAS
WHILE SFC LOW PRESSURE OVER SOUTHEAST COLORADO WITH A SFC BOUNDARY
EXTENDING NORTHEASTWARD ACROSS NORTHERN KANSAS AND INTO IOWA. RADAR
MOSAIC SHOWS A BAND OF INCREASING REFLECTIVITY ACROSS NORTHERN
NEBRASKA AND NORTHWEST IOWA WITH WEAKER RETURNS EXTENDING ALL THE
WAY INTO WESTERN WISCONSIN IN CONJUNCTION OF WELL DEFINED H7-H6 FN
VECTOR DIVERGENCE AND ENTRANCE REGION OF 120+ KT UPPER JET BEING
PULLED ALONG THE IA/MN BORDER.
THE INITIAL FORECAST CONCERN IS THE APPROACHING DAKOTAS TROUGH AND
THE SFC INVERTED TROUGH/SFC BOUNDARY THAT GETS KICKED NORTHEASTWARD
AHEAD OF IT. NAM AND GFS ARE BOTH INDICATING THAT THE MID LEVEL
FRONTOGENESIS WILL STAY JUST TO THE SOUTH OF THE CWA IN IOWA THIS
MORNING. LATEST RADAR TRENDS SUPPORT THIS. POTENTIAL VORTICITY ANOMALY
ASSOCIATED WITH THE APPROACHING TROUGH WILL CONTINUE TO PUSH
SOUTHWARD AND EASTWARD ENOUGH SO TO SUGGEST THAT THE MID LEVEL
BAROCLINIC ZONE AND FRONTOGENETICAL BAND WILL STAY JUST SOUTH OF THE
CWA AS THE GFS AND NAM INDICATE. 300K ISENTROPIC SFC PROGS KEEP
CONDENSATION PRESSURE DEFICITS HIGH ACROSS THE SOUTHERN CWA AS WOULD
BE EXPECTED AND ISENTROPIC ADIABATIC OMEGA IS FOCUSED OVER CENTRAL
AND NORTH CENTRAL IOWA THIS MORNING. WITH ALL OF THAT WILL GO AHEAD
AND TRIM POPS BACK IN THE SOUTH THIS MORNING AND CONFINE THEM TO THE
SOUTHERN TIER OF COUNTIES ALONG THE IA BORDER. THE AFTERNOON BECOMES
A BIT MORE TRICKY WITH THE UPPER TROUGH AXIS PASSING THE AREA AND
ENTRANCE REGION OF THE UPPER JET MOVING OVERHEAD AS WELL. HEIGHT
FALLS INCREASE WITH THE TROUGH WHILE POOL OF -21 TO -23 C H5 AIR
SETTLES IN BY LATE AFTERNOON WITH MID LEVEL MOISTURE INCREASING AS
WELL PER TIME HIGHT SECTIONS IN CONJUNCTION WITH FORECAST H5-H3
Q-VECTOR CONVERGENCE. SUNDAY AFTERNOON SAW SOME SHOWERS ACROSS
MONTANA AHEAD OF THE WAVE AS WELL. THUS ANTICIPATE ISOLATED TO
SCATTERED SHOWERS AND PERHAPS A RUMBLE OF THUNDER /GIVEN A SMALL
AMOUNT OF ELEVATED CAPE IN THE FORECAST SOUNDINGS/ BY MID/LATE
AFTERNOON. BEST RAIN CHANCES WILL REMAIN IN PROXIMITY TO THE
ACTIVITY ACROSS THE SOUTHERN SOUTH WHICH MAY SHIFT A BIT
NORTHEASTWARD DURING THE AFTERNOON SUPPORTING SLIGHTLY HIGHER POPS
FROM RED WING TO EAU CLAIRE. CIRRUS SHIELD SHOULD BE PRETTY
EXPANSIVE THROUGH THE MIDDLE AFTERNOON WITH CU/STRATOCU ALSO
DEVELOPING THERE FOR TEMPERATURES WILL STRUGGLE TO BREAK
60...ESPECIALLY SOUTH WHERE MID/UPPER 50S SHOULD DO IT FOR HIGHS.
HIGH PRESSURE RIDGE WILL BE SQUARELY OVERHEAD WITH CLEARING SKIES
TONIGHT. GOOD RADIATIONAL COOLING CONDITIONS WILL SET UP RESULTING
IN OVERNIGHT LOWS IN THE UPPER 30S TO AROUND 40 WITH PATCHY FROST
POSSIBLE. THE HIGH SHIFTS EAST TUESDAY WITH PRESSURE GRADIENT
TIGHTENING IN THE WEST BY TUESDAY EVENING. UPPER FLOW WILL BE FAIRLY
FLAT DURING THIS TIME WHILE A SHARP LONGWAVE TROUGH PUSHES INTO THE
WESTERN CONUS SPAWNING SFC CYCLOGENESIS IN THE LEE OF THE CENTRAL
AND NORTHERN U.S. ROCKIES BY WEDNESDAY MORNING. STRONG SOUTHERLY
WINDS WILL BE IN PLACE LOCALLY FOR WEDNESDAY AS THE PRESSURE
GRADIENT TIGHTENS. WARM ADVECTION COMMENCES INTO WEDNESDAY WITH H85
TEMPS BEING BOOSTED TO AROUND +12C BY WEDNESDAY EVENING. THE MAIN
LONGWAVE AXIS /WHICH WILL BE POSITIVELY TILTED/ REMAINS TO THE WEST
BUT A SERIES OF LEAD SHORTWAVES WILL PROPAGATE EAST-NORTHEAST IN THE
FAST UPPER FLOW REACHING THE CWA WEDNESDAY EVENING. SFC LOW PRESSURE
WILL TRACK ACROSS THE DAKOTAS WITH FRONTAL BOUNDARY/INVERTED TROUGH
MOVING EASTWARD ACROSS THE CWA INTO THURSDAY MORNING. PREVIOUS
FORECAST OF LOW CHANCE POPS WEDNESDAY AND HIGH CHANCE POPS FOR
WEDNESDAY NIGHT LOOKS ON TRACK. HIGH PRESSURE LOOKS ON TRACK FOR
FRIDAY BEHIND THE LOW. LOW CONFIDENCE BEYOND THAT WITH THE MODELS
DISAGREEING ON THE TRACK OF TROPICAL CYCLONE IKE BUT IT DOES APPEAR
THAT ANOTHER UPPER TROUGH WILL BE APPROACHING SATURDAY SO WILL
CONTINUE WITH CONTINUITY AND KEEP LOW CHANCE POPS FOR THAT TIME.
&&
.MPX WATCHES/WARNINGS/ADVISORIES...
MN...NONE.
WI...NONE.
&&
$$
RAH/MDB
