285 
FXUS63 KUNR 181130
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
530 AM MDT WED APR 18 2012

.AVIATION.../12Z TAFS/
VFR CONDITIONS WILL PREVAIL IN THE PERIOD. ANOTHER MID LEVEL
IMPULSE WILL TRAVERSE THE REGION...SUPPORTING SCATTERED SHOWERS
AND ISOLATED THUNDERSTORMS THIS AFTERNOON AND EVENING...WITH MVFR
VIS POSSIBLE IN ANY SHRA. SOUTHERLY WINDS WILL VEER NORTHERLY AT
THE TERMINALS WITH THE PASSAGE OF THE UPPER IMPULSE AND SFC
TROUGH.

&&

.PREV DISCUSSION... /ISSUED 346 AM MDT WED APR 18 2012/ 

DISCUSSION...WATER VAPOR IMAGERY SHOWS NEAR ZONAL FLOW EXTENDING
ACROSS THE NORTHWESTERN AND NORTH CENTRAL CONUS AS YESTERDAYS
SHORTWAVE DEPARTS INTO THE UPPER MIDWEST. THE NEXT WAVE
APPROACHING WAVE IS CROSSING THE NORTHERN ROCKIES AT THIS TIME.
SURFACE ANALYSIS SHOWS WEAK HIGH PRESSURE AXIS CENTERED OVER THE
CWA. TODAY...THE NEXT WAVE WILL REACH THE BY THIS AFTERNOON.
MODELS HAVE SLOWED DOWN THE TIMING A BIT...SO HAVE REMOVED POPS
FOR THIS MORNING. SHOWERS ARE AGAIN EXPECTED TO DEVELOP THIS
AFTERNOON INTO THE EVENING AS THE WAVE MARCHES THROUGH. WITH
MARGINAL INSTABILITY...SOME ISOLATED THUNDERSTORMS ARE EXPECTED.
SHOWERS WILL PUSH EASTWARD TONIGHT AS A COLD FRONT CROSSES THE
AREA...MAINLY AFFECTING CENTRAL SOUTH DAKOTA AFTER MIDNIGHT.

CYCLONIC FLOW ALOFT WILL CONTINUE THURSDAY AS THE NEXT SERIES OF
WAVES CROSS THE AREA. SLIGHTLY COOLER TEMPERATURES ARE
EXPECTED...WHICH ARE CLOSER TO SEASONAL AVERAGE HIGHS IN THE 50S.
CHANCES FOR SHOWERS WILL CONTINUE THROUGH THE DAY AND
EVENING...THEN ENDING FROM WEST TO EAST AS UPPER RIDGING BUILDS IN
AS THE SHORTWAVE AXIS PUSHES INTO THE MISSOURI VALLEY. WITH
RIDGING IN PLACE FRIDAY...QUIET WEATHER IS EXPECTED. WARM AIR
ADVECTION FROM THE SOUTHWEST LATE FRIDAY WILL BE DELAYED...SO ONLY
A SLIGHT RISE IN TEMPS ARE EXPECTED COMPARED TO HIGHS THURSDAY.
CHANCES OF SHOWERS WILL RETURN FRIDAY NIGHT AS A SHORTWAVE
CROSSES THE NORTHERN PLAINS. HOWEVER CHANCES FOR SHOWERS WILL BE
MINIMAL AS THE MAIN WAVE CROSSES NORTH DAKOTA INTO EAST RIVER SD. 

EXTENDED...

SATURDAY THROUGH TUESDAY

DEEP LAYER RIDGING SPURRED BY A SPLITTING LONGWAVE EASTERN PAC 
TROUGH WILL DOMINATE THROUGH THE MUCH OF THE PERIOD. HOWEVER...THERE 
ARE STRONG INDICATIONS FOR A CONTINUED PROGRESSIVE PATTERN GIVEN THE 
CONTINUED AGGRESSIVE NORTHERN STREAM...COUPLED WITH THE EMINENT 
BREAKDOWN OF THE NORTH ATLANTIC RIDGE. PARADE OF POTENT NORTH PAC 
UPPER IMPULSES WILL SUPPORT A PASSIVE WARM SPELL IN THE 
PERIOD...WITH RAIN CHANCES ARRIVING BY TUE-WED WITH THE PASSAGE A OF 
RATHER STRONG MID LEVEL IMPULSE...SUPPORTING A TRANSITION BACK TO 
PERTURBED WNW LOCAL FLOW.

SAT-MON...HEIGHT RISES PER A WAA REGIME EXPECTED WITH A DEPARTING 
SFC WAVE SUPPORTING A TRANSITION TO WESTERLY DOWNSLOPE WARMING FLOW 
SAT. LL FLOW WILL THEN TURN SE SUN AS SFC PRESSURE FALLS ENSUE PER 
THE ADVECTING NORTHERN STREAM SPLITTING COUNTERPART TO THE EASTERN 
PAC LONGWAVE TROUGH. THE GFS SUGGESTS FLOW WILL THEN TURN WESTERLY 
MON AS A LEAD SFC /THERMAL/ TROUGH ADVECTS THROUGH THE REGION. 
HOWEVER...THE ECMWF SUPPORTS CONTINUED SE FLOW INTO TUES. 
HENCE...REMAINED CONSERVATIVE PER HIGHS SUN-MON GIVEN SE FLOW 
CONCERNS...WITH A CONTINUED UP TREND IN TEMPS TUES GIVEN 
DOWNSLOPE FLOW INDICATIONS WITH H85 T/S AT 20-25C. 

TUES-WED...GIVEN PREFERENCE TOWARD A STRONG NORTHERN STREAM 
INFLUENCE...HAVE ADDED LOW CHANCE POPS TUE-WED...WHICH COVER PRIMARY 
WAVE TIMING CONCERNS AS WELL AS A TRANSITION TO ACTIVE WNW FLOW. 
CURRENT ECMWF/GFS/UKMET MID LEVEL HEIGHT/FLOW PROGS SUGGEST A 
RATHER INTENSE UPPER LEVEL WAVE...WHICH GIVEN SUFFICIENT LL MOISTURE 
RETURN COULD SUPPORT A STRONG CONVECTIVE RESPONSE. GIVEN THE 
POTENTIAL INTENSITY OF THIS SYSTEM...STRONGLY TRENDED TEMPS BACK TO 
AROUND CLIMO NORMS BY WED.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...NONE.
WY...NONE.
&&

$$

SHORT TERM...13
LONG TERM...JC
AVIATION...JC