Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202346
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
646 PM CDT MON APR 20 2015

.SHORT TERM /TONIGHT/...
ISSUED AT 302 PM CDT MON APR 20 2015

MAIN CHALLENGES OVERNIGHT WILL BE TEMPERATURES. EARLY AM SURFACE
ANALYSIS SHOWS THREE DISTINCT COLD FRONTS...WITH THE FIRST HAVING
PASSED THROUGH THE REGION SUNDAY...THE SECOND IN FAR SOUTHERN IOWA
AT 12Z AND A THIRD OVER SOUTHERN MN AT 12Z WHICH WILL SLOWLY EDGE
SOUTH OVERNIGHT TONIGHT AS THE DEEPENING LOWS OVER LAKE MICHIGAN
EVENTUALLY MERGE INTO ONE SYSTEM NEAR HUDSON BAY CANADA BY 06Z
TUESDAY. THIS WILL PULL THE FINAL FRONT SOUTH INTO IOWA BY TUESDAY.
MEANWHILE...STRONG WINDS OF THIS AFTERNOON WILL DECOUPLE TONIGHT AND
CUMULUS FIELD CAUSED BY THE H850 COLD AIR ADVECTION WILL DISSIPATE
WITH TIME...LEAVING THE AREA GENERALLY CLEAR TO PARTLY CLOUDY. DRIER
MID LEVEL AIR BEGINNING TO MOVE INTO WESTERN IA THIS AFTERNOON WITH
SHOWERS ON THE DECREASE. CLEARING WILL ALLOW FOR SOME RADIATIONAL
COOLING AND TEMPERATURES TO DROP INTO THE 30S OVERNIGHT OVER ALL OF
THE FORECAST AREA. DESPITE THE LOW MINIMUMS EXPECTED...GRADIENT
WINDS ARE LIKELY TO REMAIN NEAR OR ABOVE 10 KTS WHICH WILL KEEP ANY
THREAT OF FROST TO A MINIMUM. THIRD COLD FRONT ARRIVING ACROSS THE
NORTH BY 08-10Z WILL CREATE NEAR ADIABATIC CONDITIONS AGAIN...
PROMOTING MIXING AND STRONGER WINDS WHICH WILL KEEP BOUNDARY LAYER
MIXED TOWARD MORNING. GROUND MOISTURE SHOULD ALSO INHIBIT FROST
FORMATION TONIGHT...AND NOT OCCUR UNTIL WINDS BECOME LIGHT ENOUGH TO
CAUSE LOWS NEAR OR UNDER 30 DEGREES...NEAR THE MID TO END OF THE
WEEK.

.LONG TERM /TUESDAY THROUGH MONDAY/...
ISSUED AT 302 PM CDT MON APR 20 2015

THE LARGE ATMOSPHERIC GYRE DEVELOPING OVER THE MINNESOTA/CANADA
BORDER TODAY WILL SLIDE OVER THE GREAT LAKES REGION ON TUESDAY AND
EVENTUALLY OVER UPPER NEW ENGLAND AND SOUTHEASTERN CANADA LATER IN
THE WEEK. THIS WILL KEEP US BENEATH A PERSISTENT BUT GRADUALLY
WEAKENING NORTHWESTERLY FLOW REGIME...LEADING TO CONTINUED COOL
WEATHER AND MOSTLY DRY CONDITIONS FOR THE REMAINDER OF THE WORK
WEEK.

ON TUESDAY...A SHORTWAVE IMPULSE ROUNDING THE LARGE CYCLONE WILL
MOVE OVER IOWA AND PUSH THROUGH A SURFACE REFLECTION IN THE FORM
OF A RENEWED WIND SURGE AND REINFORCEMENT OF THE COLD AIR IN
PLACE. MOST OF THE PROGNOSTIC MODELS ARE DEPICTING A BAND OF
CLOUDS AND LIGHT RAIN DEVELOPING ALONG THIS FEATURE AS IT MOVES
THROUGH...ESPECIALLY IN THE AFTERNOON AND EVENING...AND THE GFS
AND NAM SOUNDINGS SHOW THIS POSSIBILITY AS WELL WITH THE
DEVELOPMENT OF A PRETTY DECENT ALBEIT HIGH BASED SATURATED LAYER.
GIVEN THE PREVALENCE OF LIGHT SHOWERS AND EVEN SOME TINY HAIL
EARLIER TODAY...BELIEVE THERE IS A DECENT CHANCE OF A LOWER
MAGNITUDE REPEAT TOMORROW AND HAVE INTRODUCED SLIGHT POPS AND RW-
MENTION FROM ABOUT 20Z-03Z TUESDAY AFTERNOON AND EVENING. IN ANY
EVENT...THE RAIN WILL BE OF LITTLE CONSEQUENCE AND STRUGGLE TO
EVEN PRODUCE MEASURABLE PRECIPITATION. HOWEVER...THE CLOUDS
ASSOCIATED WITH THIS FEATURE...AS WELL AS THE RENEWED SHOT OF COLD
AIR ADVECTION BEHIND THE REINFORCING WIND SURGE...WILL HELP TO
HOLD TEMPERATURES DOWN AND HAVE COOLED OFF THE MAX TEMP FORECAST
ACCORDINGLY. WINDS WILL BE PRETTY STRONG BEHIND THE
SURGE...ESPECIALLY ACROSS OUR NORTHERN AND NORTHWESTERN
COUNTIES...AND ANOTHER WIND ADVISORY MAY BECOME NECESSARY FOR
TUESDAY. THIS WILL BE CLOSELY MONITORED ON TONIGHTS UPDATE SHIFTS.

BY TUESDAY NIGHT AS THE LARGE GYRE FINALLY BEGINS TO MOVE OFF TO
OUR EAST/NORTHEAST...IT WILL ALLOW A LARGE COLD SURFACE HIGH
PRESSURE RIDGE TO SPILL DOWN ACROSS THE UPPER MIDWEST AND IOWA FOR
A COUPLE OF DAYS. THIS WILL RESULT IN A NOTABLE DIMINISHING OF
SURFACE WINDS...BUT WILL ALSO CLEAR OUT THE CLOUDS AND ALLOW FOR
MORE EFFICIENT RADIATIONAL COOLING DURING THE OVERNIGHTS. THIS
POTENTIAL SHOULD BE MAXIMIZED ON WEDNESDAY NIGHT/THURSDAY
MORNING...THEN THE RIDGE AXIS WILL BE NEARBY AND MOST AREAS WILL
SEE THEIR COLDEST TEMPERATURES OF THE WEEK. THIS WILL LIKELY
RESULT IN THE CONSIDERATION OF FROST/FREEZE HEADLINES AT SOME
POINT...OR POSSIBLY MULTIPLE POINTS...THIS WEEK.

BY THURSDAY NIGHT THE INFLUENCE OF THE DEPARTING CYCLONE OVER IOWA
WILL BE VIRTUALLY NIL...BUT A MUCH WEAKER MID/UPPER LEVEL TROUGH
WILL BE APPROACHING FROM THE FOUR CORNERS REGION. THE SUBSEQUENT
EVOLUTION OF THIS FEATURE HAS BEEN THE OBJECT OF MUCH HAND
WRINGING BY THE LONG RANGE MODELS OVER THE LAST FEW DAYS...WITH
CONSIDERABLE INCONSISTENCY AND VARIABILITY IN WHAT THEY WANT TO DO
WITH THE LOW AS IT MOVES UP OVER THE MIDWEST. THIS SITUATION HAS
NOT IMPROVED TODAY...AND HAVE STAYED CLOSE TO PREVIOUS FORECAST
AND MODEL CONSENSUS FOR THE WEEKEND FORECAST. IN GENERAL THE MOST
LIKELY SCENARIO IS SHOWERS AND A FEW THUNDERSTORMS MAINLY ACROSS
THE SOUTHERN HALF OF THE STATE BETWEEN FRIDAY NIGHT AND SATURDAY
NIGHT...BUT TO STATE ANYTHING WITH MORE DETAIL THAN THAT AT THIS
POINT WOULD BE FOLLY. THE GOOD NEWS IS THAT NONE OF THE SOLUTIONS
RESULT IN MUCH POSSIBILITY OF HAZARDOUS WEATHER ACROSS IOWA. THE
BAD NEWS IS THAT NONE OF THEM BRING MUCH WARMING EITHER.

&&

.AVIATION...21/00Z
ISSUED AT 641 PM CDT MON APR 20 2015

STRONG NORTHWEST GRADIENT ACROSS THE TAF SITES. WINDS WILL
DECOUPLE AT THE SURFACE AND BECOME LIGHT AFTER SUNSET. A COLD
FRONT WILL MOVE INTO NORTHERN IOWA LATE TONIGHT...AND ACROSS THE
STATE TUESDAY MORNING. STRONG NWLY WINDS ACROSS THE SITES TUESDAY
MID MORNING THROUGH THE DAY WITH ISOLATED SHOWERS.

OUTLOOK...HIGH PRESSURE WILL BUILD INTO THE AREA TUESDAY NIGHT
WITH VFR CONDITIONS THROUGH THE REST OF THE WORK WEEK.

&&

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

&&

$$

SHORT TERM...REV
LONG TERM...LEE
AVIATION...MS APR 15



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