Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 261732
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1132 AM CST THU FEB 26 2015

.SHORT TERM /TODAY/...
ISSUED AT 335 AM CST THU FEB 26 2015

ARCTIC HIGH PRESSURE WILL SETTLE SOUTHEAST INTO THE DAKOTAS TODAY
WITH COLD ADVECTION PREVAILING THROUGH THE DAY ACROSS IOWA. WIND
CHILL VALUES OF 20 TO 25 BELOW WILL BE COMMON ACROSS THROUGH
NORTHWEST PORTION OF THE FORECAST AREA THROUGH MID MORNING. SOME
IMPROVEMENT IN WIND CHILLS THIS AFTERNOON THOUGH VALUES WILL REMAIN
QUITE COLD IN THE 5 TO 15 BELOW RANGE. A SHORT WAVE IS DROPPING
SOUTH THROUGH MINNESOTA THIS MORNING AND WILL BE ARRIVING WITH VERY
DRY AIR IN PLACE. STILL THE COMBINATION OF THE COLD ADVECTION AND
WEAK FORCING COULD PRODUCE A FEW FLURRIES OR LIGHT SNOW SHOWERS OVER
THE NORTHEAST THIS AFTERNOON. THESE CLOUDS AND PASSING FLURRIES VERY
WELL MAY BE THE RESULT OF PASSING ICE CRYSTALS WITH THE ARRIVAL OF
ARCTIC COLD.

.LONG TERM /TONIGHT THROUGH WEDNESDAY/...
ISSUED AT 335 AM CST THU FEB 26 2015

FROM TONIGHT INTO SATURDAY THE BIG STORY WILL BE VERY COLD
TEMPERATURES ACROSS THE AREA AS A LARGE HIGH PRESSURE AREA
DOMINATES WITH FRESH SNOW ON THE GROUND ALLOWING TEMPERATURES TO
RADIATE TO NEAR RECORD LEVELS AT NIGHT. THE CENTER OF THE HIGH
PRESSURE AREA WILL SLIDE ACROSS CENTRAL IOWA ON FRIDAY AND ITS
INFLUENCE WILL LINGER ESPECIALLY ACROSS OUR EASTERN COUNTIES ON
FRIDAY NIGHT...THEREFORE HAVE CONTINUED TREND OF PREVIOUS SHIFTS
IN FORECASTING TEMPERATURES BELOW GUIDANCE IN THESE TWO TIME
FRAMES/AREAS. THE RESULT IS LOWS IN THE TEENS BELOW ZERO NORTH AND
NORTHEAST BOTH TONIGHT AND FRIDAY NIGHT...WITH HIGHS ON FRIDAY
LIKELY IN THE SINGLE DIGITS. THIS WILL UNDOUBTEDLY REQUIRE
ADDITIONAL WIND CHILL HEADLINES BOTH NIGHTS...HOWEVER WITH WINDS
BEING FAIRLY LIGHT AT LESS THAN 10 MPH THE EFFECTS WILL BE MORE
DUE TO ABSOLUTE TEMPERATURE. WILL LEAVE THESE HEADLINE DECISIONS
TO THE DAY SHIFT AS ANY PRODUCTS FOR TONIGHT WILL BE BETTER RAISED
AFTER THE CURRENT ADVISORY DISCUSSED IN THE SHORT TERM SECTION
EXPIRES.

A PATTERN CHANGE WILL BEGIN THIS WEEKEND AS IS EVIDENT IN FORECAST
500 MB FIELDS. A LARGE TROUGH MOVING ACROSS THE WESTERN U.S. WILL
BREAK UP WITH A NORTHERN STREAM COMPONENT INTENSIFYING AND
SWEEPING ACROSS THE UPPER MIDWEST FROM SATURDAY NIGHT THROUGH
SUNDAY...WHILE THE SOUTHERN COMPONENT WILL MEANDER NEAR THE
SOUTHERN CALIFORNIA COAST INTO EARLY NEXT WEEK AND BE A PLAYER IN
OUR WEATHER LATER ON. THE FIRST...NORTHERN STREAM TROUGH AS IT
APPROACHES ON SATURDAY WILL INDUCE THE DEVELOPMENT OF AN INVERTED
TROUGH AT THE SURFACE...STRETCHING ROUGHLY ACROSS KANSAS AND
SOMEWHERE NEAR THE IOWA/MISSOURI BORDER BY SATURDAY NIGHT. THIS
FEATURE WILL ALLOW FOR GOOD LOW LEVEL MOISTURE ADVECTION EVEN AS A
LARGE AREA OF ASCENT ASSOCIATED WITH THE APPROACHING 500 MB TROUGH
MOVES OVERHEAD. WITHIN THIS REGIME THERE HAS BEEN BROAD CONSENSUS
AMONG THE PROGNOSTIC MODELS FOR THE LAST COUPLE OF DAYS IN
DEVELOPING A LARGE SWATH OF PRECIPITATION AROUND THE SURFACE
TROUGH/FRONTAL ZONE...STRETCHING UP FROM KANSAS ACROSS MUCH OF
MISSOURI AND IOWA...FROM ABOUT SATURDAY AFTERNOON INTO SUNDAY.
WHILE THERE REMAINS SOME UNCERTAINTY IN BEGINNING AND ENDING TIMES
OF PRECIPITATION ACROSS OUR FORECAST AREA...PRECIPITATION IS
BECOMING INCREASINGLY LIKELY DURING THE SATURDAY AFTERNOON INTO
SATURDAY NIGHT TIME FRAME IN PARTICULAR AND HAVE INCREASED POPS
INTO THE 70 TO 80 RANGE ACCORDINGLY. SUNDAY IS A BIT MORE
PROBLEMATIC AS THE PASSAGE OF THE 500 MB TROUGH AXIS PUSHES THE
SURFACE BOUNDARY SOUTHEASTWARD AND CLEARS THE PRECIPITATION OUT OF
OUR AREA...WITH THE TIMING OF THIS CLEARING STILL BEING UP FOR
DEBATE BUT GENERALLY SOMETIME DURING THE DAY ON SUNDAY.

IN TERMS OF THE PRECIPITATION TYPE AND AMOUNT FOR THE WEEKEND
SYSTEM...FORECAST SOUNDINGS AND EXPECTED TEMPERATURES DICTATE ALL
SNOW WITH PERHAPS JUST A BRIEF WINDOW OF OPPORTUNITY FOR SOME
MIXED PRECIP IN OUR SOUTHERN COUNTIES ON SATURDAY NIGHT. THERE
REMAINS SOME SIGNAL FOR A CONVECTIVE PRECIP BAND NEAR THE LOW
LEVEL TROUGH/FRONTAL BOUNDARY ON SATURDAY NIGHT AND SUNDAY...BUT
MOST SOLUTIONS HAVE BEEN PLACING THIS FEATURE JUST SOUTH OF OUR
AREA ACROSS NORTHEASTERN KANSAS AND NORTHERN MISSOURI. AT ANY
RATE...SOUNDINGS INDICATE THAT WITH THE INITIAL PHASE OF THE
SYSTEM SPREADING PRECIPITATION ACROSS MOST OF IOWA FROM SATURDAY
AFTERNOON INTO SATURDAY NIGHT...THERE WILL BE A DEEP SATURATED
LAYER BETWEEN ABOUT -10 TO -15 C INDICATING THE POSSIBILITY OF A
FAIRLY PROLONGED DEEP DENDRITIC GROWTH ZONE EVENT. WHILE FORCING
IS MODEST AT TIMES AND SOMEWHAT ABOVE THIS GROWTH ZONE...THERE IS
NEVERTHELESS A GOOD CHANCE FOR EFFICIENT ACCUMULATING SNOWFALL
LATE SATURDAY ALTHOUGH CERTAINLY THERE IS STILL SOME WIGGLE ROOM
IN THE SOLUTIONS FOR THE OVERALL TRACK AND THERMAL PROFILES
ASSOCIATED WITH THIS SYSTEM. IT WILL BEAR WATCHING OVER THE NEXT
COUPLE OF DAYS AS WE HONE IN ON SNOWFALL AMOUNTS AND POSSIBLY
HEADLINE ISSUANCES.

IN WAKE OF THE WEEKEND SYSTEM ANOTHER HIGH PRESSURE AREA WILL
SLIDE QUICKLY ACROSS THE REGION ON SUNDAY NIGHT...BUT BY MONDAY
OUR OLD FRIEND THE LEFTOVER SOUTHERN STREAM 500 MB TROUGH THAT
WILL HAVE BEEN MEANDERING ACROSS THE SOUTHWESTERN U.S. ALL WEEKEND
WILL RETURN TO THE PICTURE. THE GFS AND ECMWF ARE FOR NOW IN GOOD
AGREEMENT IN THE EVOLUTION OF THE 500 MB PATTERN DURING THIS
TIME...AS A NEW TROUGH DIGGING DOWN THE PACIFIC U.S. COAST WILL
EJECT THE ENERGY FROM THE OLD CUTOFF TROUGH NORTHEASTWARD ACROSS
THE CENTRAL U.S. IN SEVERAL WAVES...BEFORE THE NEWLY RECONSTITUTED
WESTERN TROUGH FINALLY SWINGS ACROSS OUR REGION AROUND WEDNESDAY
OR THURSDAY. AS THE INITIAL IMPULSE EJECTED OUT OF THE BASE OF THE
TROUGH ARRIVES OVER IOWA IT WILL BE HERALDED BY A TURN TO
SOUTHWESTERLY 500 MB FLOW OVERHEAD...WITH A SURFACE LOW PRESSURE
DEVELOPING RAPIDLY OVER KANSAS ON MONDAY NIGHT AND SPREADING A
LARGE PRECIPITATION SHIELD ALONG ITS FORWARD FLANK ACROSS IOWA AND
SURROUNDING STATES. WHILE THE FORWARD SPEED OF THIS SYSTEM
DICTATES A FAIRLY BRIEF PERIOD OF PRECIPITATION...IT WILL BE
DRAWING UP WARMER AIR AND MOISTURE FROM THE SOUTH AND APPEARS TO
BE A PRETTY DYNAMIC SETUP MORE AKIN TO WHAT WE WOULD SEE LATER IN
THE SPRING. THIS LEADS TO A POTENTIAL FOR HIGHER QPF...HOWEVER
ROUGH THERMAL FIELDS SHOW MORE OF A MIXED PRECIPITATION EVENT THAN
ALL SNOW AT THIS TIME SO THE IMPACTS ARE IN FLUX. HAVE INCREASED
POPS TO LIKELY ACROSS MOST OF THE AREA ON MONDAY NIGHT BUT
INTRODUCED SOME MIXED PRECIPITATION WORDING MAINLY ACROSS ABOUT
THE SOUTHERN HALF OF THE AREA. IT WILL TAKE SOME TIME BEFORE THE
DETAILS OF THIS SYSTEM CAN BE WORKED OUT WITH BETTER ACCURACY.
SIMILARLY...ANOTHER ROUND OF PRECIPITATION WILL BE POSSIBLE AS THE
LARGER TROUGH KICKS ACROSS THE AREA RIGHT AROUND THE END OF THE
SEVEN DAY FORECAST PERIOD...BUT AS OF NOW THE DETAILS OF SENSIBLE
WEATHER ASSOCIATED WITH THIS PROGRESSION ARE STILL HIGHLY
UNCERTAIN.

&&

.AVIATION...26/18Z
ISSUED AT 1132 AM CST THU FEB 26 2015

SOME PATCHY BLOWING SNOW AND ICE CRYSTALS IN THE AIR WILL GIVE
A FEW-SCT LIFR TO IFR DECK THIS AFTERNOON AND MAY BRIEFLY REDUCE
VSBYS TO 1 TO 3 MILES. OTHERWISE VFR CONDITIONS WITH DIMINISH
WINDS EXPECTED TONIGHT INTO FRIDAY.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...DONAVON
LONG TERM...LEE
AVIATION...BEERENDS


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