Prognostic Meteorological Discussion
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FXUS21 KWNC 191954
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 19 2017

SYNOPSIS: DURING WEEK-1, MID-LEVEL LOW PRESSURE IS FORECAST OVER THE WESTERN
U.S. WITH MID-LEVEL HIGH PRESSURE OVER THE GREAT LAKES. A COLD FRONT IS
FORECAST TO PUSH EASTWARD AHEAD OF THE MID-LEVEL LOW PRESSURE AND INTO THE
GREAT LAKES BY THE END OF WEEK-1. THE PATTERN IS FORECAST TO SHIFT EASTWARD IN
WEEK-2, WITH MID-LEVEL HIGH PRESSURE BUILDING INTO THE WEST. HURRICANES MARIA
MAY POSE A THREAT TO THE EASTERN U.S. DURING THE FORECAST PERIOD. PERIODIC
SURFACE LOW PRESSURE SYSTEMS ARE FORECAST IN THE GULF OF ALASKA THROUGHOUT THE
OUTLOOK PERIOD.

HAZARDS

HEAVY SNOW ACROSS PORTIONS OF WYOMING AND MONTANA, FRI, SEP 22.

HEAVY RAIN ACROSS PORTIONS OF THE GREAT PLAINS, AND THE UPPER MISSISSIPPI
VALLEY, FRI-SUN, SEP 22-SEP 24.

HEAVY RAIN ACROSS PORTIONS OF THE SOUTHERN PLAINS, MON-TUE, SEP 25-SEP 26.

HIGH SIGNIFICANT WAVE HEIGHTS FOR CAPE HATTERAS AND THE OUTER BANKS, TUE, SEP
26.

MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI
VALLEY, THE GREAT LAKES, AND THE UPPER MISSISSIPPI VALLEY, FRI-SUN, SEP 22-SEP
24.

MUCH BELOW NORMAL TEMPERATURES MOVING ACROSS MANY PARTS OF THE WEST, FRI-TUE,
SEP 22-SEP 26.

HEAVY RAIN ACROSS PORTIONS OF THE KENAI PENINSULA, SOUTHEAST MAINLAND ALASKA,
AND THE ALASKA PANHANDLE, SUN, SEP 24.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHEAST.

HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE SOUTHERN PLAINS, WED,
SEP 27.

HIGH SIGNIFICANT WAVE HEIGHTS FOR CAPE HATTERAS AND THE OUTER BANKS, WED, SEP
27.

SEVERE DROUGHT ACROSS PARTS OF THE NORTHERN ROCKIES, NORTHERN AND CENTRAL GREAT
PLAINS, CORN BELT, MINNESOTA, RIO GRANDE VALLEY, AND HAWAII.

DETAILED SUMMARY

FOR FRIDAY SEPTEMBER 22 - TUESDAY SEPTEMBER 26: THE FORECAST PATTERN THROUGHOUT
WEEK-1 CONTINUES TO BE AMPLIFIED, AND A STARK DEPARTURE FROM THE PAST SEVERAL
MONTHS, WITH A POSITIVELY TILTED 500-HPA TROUGH OVER THE WESTERN U.S. AND
RIDGING OVER THE GREAT LAKES. BEHIND THE ANOMALOUS TROUGH, MUCH BELOW-NORMAL
TEMPERATURES ARE POSSIBLE (MINIMUM TEMPERATURE ANOMALIES OF 12-16 DEGREES F
BELOW NORMAL) THROUGHOUT WEEK-1 FOR MUCH OF THE WEST. THE HIGHLIGHTED AREAS
COINCIDE WITH REGIONS THAT EXHIBIT THE BEST POTENTIAL FOR A FROST OR FREEZE TO
OCCUR OVERNIGHT DURING THE PERIOD. RIDGING ACROSS THE GREAT LAKES FAVORS MUCH
ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY, THE
GREAT LAKES, AND THE UPPER MISSISSIPPI VALLEY SEP 22 TO 24. MAXIMUM TEMPERATURE
DEPARTURES IN THIS AREA MAY REACH 16 TO 20 DEGREES F ABOVE NORMALS, WITH
TEMPERATURES IN THE HIGH 80S AND LOW 90S (DEG F).



HEAVY SNOW (EXCEEDING 6" IN 24 HOURS) IS POSSIBLE ACROSS HIGHER ELEVATIONS OF
PORTIONS OF WYOMING AND MONTANA ON SEP 22 IN ADVANCE OF A COLD FRONT PUSHING
SOUTHWARD. AS THIS COLD FRONT ADVANCES EASTWARD, ANOMALOUS SOUTHERLY FLOW AT
THE LOW LEVELS WILL HELP PULL MOISTURE NORTHWARD OUT OF THE GULF OF MEXICO AND
INTO THE CENTRAL CONUS. THIS MOISTURE, COUPLED WITH FORCING ALONG THE ADVANCING
COLD FRONT, IS FORECAST TO GIVE RISE TO A SLOW-PROGRESSING HEAVY RAIN EVENT
(DAILY TOTALS EXCEEDING 1 INCH, EVENT TOTALS THAT COULD APPROACH 6 INCHES)
DURING SEP 22-24 FOR THE GREAT PLAINS AND THE UPPER MISSISSIPPI VALLEY. HEAVY
RAIN IS EXPECTED TO SHIFT SOUTHWARD TO THE SOUTHERN PLAINS SEP 25 TO 26 AS THE
COLD FRONT SINKS SOUTHWARD. ATMOSPHERIC CONDITIONS MAY SUPPORT AN INCREASED
LIKELIHOOD FOR SEVERE WEATHER ON SEP 22 ACROSS PARTS OF MINNESOTA AND THE
SURROUNDING AREA, FOLLOWED BY PARTS OF THE HIGH PLAINS ON MONDAY AND TUESDAY.
HOWEVER, DUE TO SUFFICIENT MODEL DIFFERENCES REGARDING SYNOPTIC FEATURES, THE
STORM PREDICTION CENTER HAS NOT HIGHLIGHTED A REGION AT THIS TIME.



THE NATIONAL HURRICANE CENTER (NHC) FORECASTS HURRICANE JOSE TO MAKE IT AS FAR
NORTH AS 40N BY WEDNESDAY, BEFORE CURVING BACK SOUTHWARD. BEYOND THIS POINT THE
WEATHER PREDICTION CENTER (WPC) FORECASTS JOSE TO LOOP BACK TO THE WEST
SLIGHTLY. NO HAZARDS ARE EXPECTED AT THIS TIME SINCE JOSE IS ANTICIPATED TO
REMAIN OFFSHORE. HURRICANE MARIA IS FORECAST TO APPROACH THE EASTERN U.S. BY
THE END OF WEEK-1. THE LATEST WPC FORECAST POINTS ON THE 25TH AND 26TH ARE
30.5N/72W AND 32N/71W RESPECTIVELY. WHILE THIS TRACK IS WELL OFF THE ATLANTIC
COAST, ASSOCIATED HIGH WIND AND SIGNIFICANT WAVE THREATS ARE POSSIBLE FOR
PORTIONS CAPE HATTERAS AND THE OUTER BANKS AREA WITH MARIA`S APPROACH. AT THIS
TIME ONLY SIGNIFICANT WAVES ARE HIGHLIGHTED AT THIS TIME, DUE TO UNCERTINTY
REGARDING THE FORECAST SUSTAINED WIND SPEEDS. INTERESTED PARTIES SHOULD
MAINTAIN THEIR SITUATION AWARENESS REGARDING MARIA BY FOLLOWING SUBSEQUENT NHC
FORECAST ADVISORIES, CPC U.S. HAZARDS OUTLOOKS, AND WPC FORECASTS FOR
INFORMATION ON MARIA`S TRACK AND IMPACTS. RESIDENTS LIVING IN THE VICINITY OF
COASTAL AREAS HIGHLIGHTED ARE REMINDED TO HAVE AN UP TO DATE HURRICANE
PREPAREDNESS PLAN TO ACT UPON, SHOULD SUBSEQUENT FORECASTS NECESSITATE IT.



RIVER FLOODING IS FORECAST TO CONTINUE FOR PARTS OF FLORIDA DUE TO ANTECEDENT
RAINFALL FROM HURRICANE IRMA.



A SURFACE LOW PRESSURE SYSTEM IS PREDICTED TO DEVELOP OVER THE GULF OF ALASKA
BY SEP 24. STRONG ONSHORE FLOW FROM THE GULF OF ALASKA IS ANTICIPATED TO BRING
HEAVY RAIN TO PORTIONS OF THE KENAI PENINSULA, SOUTHEAST MAINLAND ALASKA, AND
THE ALASKA PANHANDLE SEP 24. LOCALIZED AREAS MAY RECEIVE UP TO 2 INCHES OR
GREATER OF RAINFALL IN A 24-HOUR PERIOD.

FOR WEDNESDAY SEPTEMBER 27 - TUESDAY OCTOBER 03: THE FORECAST PATTERN IN WEEK-2
IS ANTICIPATED TO BECOME MORE PROGRESSIVE, WITH 500-HPA RIDGING BUILDING INTO
THE PACIFIC NORTHWEST WITH DEAMPLIFICATION DOWNSTREAM OVER THE THE REST OF THE
CONUS. WITH THE AMPLIFIED TROUGH STILL FORECAST ACROSS THE WEST EARLY IN THE
PERIOD, CONTINUED HEAVY RAIN IS FORECAST ACROSS PORTIONS OF THE SOUTHERN PLAINS
ON SEP 27. THE 6Z GEFS ENSEMBLE MEAN INDICATES OVER AN INCH IN A 24-HOUR
PERIOD, WHILE THE 6Z DETERMINISTIC GFS SHOWS 4 INCHES OR GREATER IN A DAY, AND
THE 0Z ECMWF DETERMINISTIC AND ECMWF ENSEMBLE DOES NOT INDICATE HEAVY
PRECIPITATION. THEREFORE IT SHOULD BE NOTED THAT THERE IS SIGNIFICANT MODEL
UNCERTAINTY ASSOCIATED WITH THIS FEATURE.



THE U.S. DROUGHT MONITOR, VALID ON SEP 12, INDICATES THAT SEVERE TO EXCEPTIONAL
(D2-D4) DROUGHT COVERS 6.24 PERCENT OF THE CONTINENTAL U.S. WITH THE WORST
CONDITIONS ACROSS EASTERN MONTANA AND THE DAKOTAS. MUCH-NEEDED RAINFALL HAS
BEEN OBSERVED ACROSS THE DROUGHT-STRICKEN AREAS OF THE NORTHERN GREAT PLAINS
SINCE THE LATEST RELEASE.

FORECASTER: MELISSA OU

$$



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