Area Forecast Discussion
Issued by NWS Pocatello, ID

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000
FXUS65 KPIH 210926
AFDPIH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE POCATELLO ID
326 AM MDT SUN SEP 21 2014

.SHORT TERM...TODAY THROUGH TUE NIGHT. LOW EXPECTED LATER TODAY IS
CURRENTLY MOVING ITS CIRRUS SHIELD INTO THE SOUTH STATE AND
APPEARS ON TIME TO PRODUCE SHOWERS AND THUNDERSTORMS FOR THE
AFTERNOON IN THE FORECAST AREA. THIS WILL INTENSIFY DURING THE
EVENING AND ONLY SUBSIDE A LITTLE FOR THE LATE NIGHT AND EARLY
MORNING. THE UPPER LEVEL LOW AND COLD AIR ALOFT PASS OVER THE
FORECAST AREA DURING THE MORNING ON MON. THIS WILL CONTINUE STRONG
INSTABILITY EVEN DURING THE OVERNIGHT...THUS HAVE CONTINUED THE
NOCTURNAL THUNDERSTORMS. MODERATE TO HEAVY RAIN EXPECTED WITH
PRECIPITABLE WATER EXCEEDING ONE INCH IN THE SNAKE RIVER PLAIN AND
EASTERN MAGIC VALLEY. WOULD BE MORE CONFIDENT OF A FLASH FLOOD
WATCH IF THERE WAS GOOD UPPER LEVEL DIFFLUENCE...BUT THERE ISNT
MUCH. THE HEAVY PRECIPITATION IS ALSO EXPECTED DURING THE
OVERNIGHT PERIODS...SO IT DOES NOT HAVE THE ADVANTAGES THAT
INSOLATION WOULD ADD TO THE CONVECTIVE MIX. THE PRIMARILY
CONVECTIVE PRECIPITATION IS ALSO FALLING IN MUCH DIFFERENT
LOCATIONS COMPARING THE GFS AND NAM. THE NAM LOOKED LIKE IT WAS
VERIFYING BETTER WITH MOISTURE ALREADY PUSHING INTO THE CENTRAL
IDAHO MOUNTAINS. SO HAVE USED THE NAM FOR THE FIRST 48
HOURS...THEN TRANSITION TO THE GFS WHICH IS MUCH DRIER FOR TUE
MORNING TO THE END OF THE NEAR TERM FORECAST. WITH THE CLOSED LOW
SITTING OVER THE TOP...DO NOT EXPECT TO SEE AND ISSUES WITH
WIND SPEED. DIRECTION WILL BE MAINLY OUT OF THE NORTH AND
NORTHEAST...SOME TIME TONIGHT SHIFTING TO SOUTHERLY. THUS MAIN
FORECAST PROBLEM IS SLOW-MOVING THUNDERSTORM ACTIVITY WITH A HIGH
AMOUNT OF AVAILABLE MOISTURE. FOR AFTERNOON HIGHS...COLDER TEMPERATURES
TODAY AND MON...BUT WARMING TREND SHOULD RETURN TUE. EXACT
OPPOSITE TRENDS FOR OVERNIGHT LOWS AS CLOUD COVER CLEARS OUT LATE
MON NIGHT. MESSICK

.LONG TERM...WED MORNING THROUGH SAT EVENING. STRONG BUT SHORT-
LIVED UPPER LEVEL RIDGING WILL HAVE ALREADY MOVED IN ON TUE AND
CONTINUES INTO WED NIGHT/THU MORNING. THE NEXT LOW IS CERTAIN ON
ARRIVING DURING THE MEDIUM RANGE FORECAST...BUT THE QUESTION IS
THE EXACT DAY AND WHETHER IT APPROACHES FROM THE WEST OR MORE FROM
THE SOUTH. GFS AND ECMWF ARE NOT AT ALL IN AGREEMENT ON WHAT SIDE
OF THE LOW WE WILL BE...AND ARE IN EVEN STRONGER DISAGREEMENT
ABOUT TIMING WITH THE ECMWF MUCH SLOWER AND SLIGHTLY DRIER. HAVE
PAINTED WITH A VERY BROAD BRUSH FOR THE THU/FRI/SAT FORECAST. THE
ONLY THING TO REMARK IS THAT IT SHOULD BE A WET PERIOD...AND IF
THE GFS IS CORRECT IT COULD SEE SNOW DEVELOPING AT VERY HIGH
ELEVATIONS...ABOVE 9000 FEET ELEVATION...IN THE FORECAST AREA.
MESSICK


&&

.AVIATION...A LOW PRESSURE SYSTEM WILL BRING SCATTERED THUNDERSTORMS
TO THE AREA THIS AFTERNOON. THESE STORMS COULD PERSIST THROUGH THE
NIGHT AND INTO MONDAY AS THE LOW MOVES THROUGH THE REGION. EXPECT
VFR CONDITIONS WITH LOCALIZED AND BRIEF MVFR/IFR CONDITIONS POSSIBLE
UNDER THUNDERSTORMS.

VALLE

&&

.FIRE WEATHER...A LOW PRESSURE SYSTEM WILL ARRIVE THIS AFTERNOON.
THIS WILL BRING SHOWERS AND THUNDERSTORMS TO THE REGION WHICH WILL
PERSIST INTO MONDAY. HIGH PRESSURE WILL RETURN FOR TUESDAY AND
PERSIST THROUGH MIDWEEK. ANOTHER LOW PRESSURE SYSTEM COULD BRING
MORE WET WEATHER TO THE AREA NEXT WEEKEND.

VALLE

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$






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