Prognostic Meteorological Discussion
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FXUS21 KWNC 211856
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 21 2017

SYNOPSIS: SURFACE HIGH PRESSURE IS FORECAST TO EXPAND SOUTH FROM CANADA INTO
THE MIDWEST, NORTHEAST, AND MID-ATLANTIC EARLY IN THE PERIOD, WHILE THE
REMNANTS OF TROPICAL STORM HARVEY TRACK ACROSS THE BAY OF CAMPECHE. UPPER-LEVEL
HIGH PRESSURE IS FORECAST TO BUILD ACROSS THE WESTERN U.S. DURING THE FINAL
WEEK OF AUGUST. UPPER-LEVEL LOW PRESSURE IS LIKELY TO REMAIN CENTERED OVER
WESTERN ALASKA AND THE ALEUTIANS THROUGH WEEK-2.

HAZARDS

HEAVY RAIN FOR THE LOWER TEXAS GULF COAST, LOWER AND MIDDLE RIO GRANDE VALLEY,
AND SOUTHERN TEXAS, THU-MON, AUG 24-28.

HEAVY RAIN FOR SOUTHERN FLORIDA, THU-FRI, AUG 24-25.

LOCALLY HEAVY RAIN FOR PARTS OF SOUTHERN COLORADO AND NEW MEXICO, THU-FRI, AUG
24-25.

MUCH ABOVE-NORMAL TEMPERATURES FOR PARTS OF THE PACIFIC NORTHWEST AND NORTHERN
ROCKIES, SUN-MON, AUG 27-28.

HEAVY RAIN SHIFTING EAST FROM THE SOUTHERN KENAI PENINSULA TO THE NORTHERN
ALASKA PANHANDLE, THU-SAT, AUG 24-26.

SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR MUCH OF THE WESTERN U.S.,
TUE-MON, AUG 29-SEP 4.

MODERATE RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PARTS OF IDAHO, OREGON, AND
WASHINGTON, TUE-WED, AUG 29-30.

FLOODING POSSIBLE FOR PARTS OF THE MIDDLE MISSISSIPPI RIVER VALLEY.

SEVERE DROUGHT ACROSS PARTS OF THE GREAT PLAINS, WESTERN CORN BELT, NORTHERN
ROCKIES, AND HAWAII.

DETAILED SUMMARY

FOR THURSDAY AUGUST 24 - MONDAY AUGUST 28: AS OF 2PM EDT ON AUGUST 21, A TROUGH
OF LOW PRESSURE (THE REMNANTS OF TROPICAL STORM HARVEY) IS APPROACHING BELIZE
AND THE YUCATAN PENINSULA. ENVIRONMENTAL CONDITIONS ARE EXPECTED TO BE MORE
FAVORABLE FOR REDEVELOPMENT OF THIS SYSTEM ONCE IT MOVES INTO THE BAY OF
CAMPECHE. THE DETERMINISTIC GFS AND ECMWF MODELS INDICATE THAT THIS SYSTEM
APPROACHES NORTHERN MEXICO OR THE LOWER TEXAS GULF COAST ON AUG 24 AND 25. THE
LOCATION THAT THIS SYSTEM MOVES ONSHORE VARIES FROM TAMPICO, MEXICO TO AS FAR
NORTH AS CORPUS CHRISTI TEXAS AMONG THE 6Z GFS ENSEMBLE MEMBERS. REGARDLESS OF
TRACK AND STRENGTH, HEAVY RAIN IS EXPECTED TO SPREAD INLAND FROM THE LOWER
TEXAS GULF COAST TO THE MIDDLE RIO GRANDE VALLEY. DUE TO VARYING MODEL
SOLUTIONS ON WHERE THE HEAVIEST RAINFALL OCCURS, A BROAD HEAVY RAIN HAZARD IS
POSTED FROM AUG 24 TO 28. GIVEN THE EXPECTED SLOW MOVEMENT OF THIS SYSTEM ONCE
IT MOVES INLAND, THE POTENTIAL EXISTS FOR VERY HEAVY RAIN AMOUNTS (5 TO 10
INCHES, OR MORE) AND FLOODING WITHIN THE OUTLINED AREA.



A TROUGH OF LOW PRESSURE IS EXPECTED TO BE LOCATED ACROSS SOUTH FLORIDA DURING
MID-WEEK. THIS TROUGH OF LOW PRESSURE AND ENHANCED MOISTURE ARE EXPECTED TO
BRING HEAVY RAIN (LOCALLY MORE THAN 2 INCHES IN 24 HOURS) TO SOUTH FLORIDA ON
AUG 24 AND 25. ALSO, CONDITIONS MAY BECOME MORE FAVORABLE TOR TROPICAL CYCLONE
DEVELOPMENT WHEN THIS TROUGH NEARS FLORIDA.



UPSLOPE FLOW BEHIND A COLD FRONT ALONG WITH ENHANCED MONSOONAL MOISTURE IS
EXPECTED TO MAINTAIN AN INCREASED COVERAGE OF THUNDERSTORMS WITH LOCALLY HEAVY
RAIN (MORE THAN 1 INCH PER 24 HOURS) ACROSS THE HIGHER TERRAIN OF SOUTHERN
COLORADO AND NEW MEXICO AND ADJACENT HIGH PLAINS THROUGH AT LEAST AUG 25.
THUNDERSTORMS WITH HEAVY RAIN WILL BE CAPABLE OF TRIGGERING FLASH FLOODING
ESPECIALLY ACROSS BURN SCAR AREAS. ELSEWHERE, HEAVY RAIN PRIOR TO THIS PERIOD
MAY CAUSE FLOODING ACROSS THE MIDDLE MISSISSIPPI RIVER VALLEY.



AN UPPER-LEVEL RIDGE IS FORECAST TO STRENGTHEN LATER THIS WEEK ACROSS THE
WESTERN U.S. WITH 500-HPA HEIGHTS INCREASING TO NEAR 588DM. MUCH ABOVE-NORMAL
TEMPERATURES ARE POSTED FOR AREAS OF THE PACIFIC NORTHWEST AND NORTHERN ROCKIES
WHERE MAXIMUM TEMPERATURES ARE FORECAST TO AVERAGE AROUND 12 DEGREES F ABOVE
NORMAL AND EXCEED 90 DEGREES F. ALSO, NUMEROUS WILDFIRES CONTINUE TO BURN IN
THESE SAME AREAS WITH THE LARGEST WILDFIRE (MORE THAN 68,000 ACRES BURNED) AT
NENA SPRINGS IN NORTHERN OREGON.



A DEEP, CLOSED LOW AT 500-HPA IS FORECAST TO DEVELOP OVER SOUTHERN MAINLAND
ALASKA DURING THE WEEKEND. THIS CLOSED LOW ALOFT IS EXPECTED TO RESULT IN
SURFACE LOW DEVELOPMENT WITH ONSHORE FLOW BRINGING HEAVY RAIN (MORE THAN 1.5
INCHES PER 24 HOURS) FROM THE SOUTHERN KENAI PENINSULA TO THE NORTHERN ALASKA
PANHANDLE FROM AUG 24 TO 26.

FOR TUESDAY AUGUST 29 - MONDAY SEPTEMBER 04: THE GFS, CANADIAN, AND ECMWF
ENSEMBLE MEANS FEATURE A HIGHLY AMPLIFIED TROUGH OVER WESTERN ALASKA AND THE
ALEUTIANS WITH ANOMALOUS RIDGING ACROSS WESTERN NORTH AMERICA DURING WEEK-2.
THIS ANOMALOUS RIDGING SUPPORTS A SLIGHT (AUG 29 AND 30) TO MODERATE (AUG
29-SEP 4) RISK OF MUCH ABOVE-NORMAL TEMPERATURES ACROSS PARTS OF THE WESTERN
U.S. THE AMPLIFIED TROUGH AND ONSHORE FLOW ARE LIKELY TO RESULT IN ABOVE-NORMAL
PRECIPITATION ACROSS SOUTHERN COASTAL ALASKA AND THE ALASKA PANHANDLE. A
CATEGORICAL HEAVY RAIN HAZARD IS NOT POSTED SINCE LOCATION AND TIMING ARE
UNCERTAIN.



HEAVY RAIN COULD LINGER INTO THE BEGINNING OF THE WEEK-2 PERIOD ACROSS THE
LOWER AND MIDDLE RIO GRANDE VALLEY, SOUTHERN TEXAS, AND THE WESTERN GULF COAST.
HOWEVER, GFS AND ECMWF ENSEMBLE MEANS GENERALLY INDICATE LESS THAN 0.5 INCH PER
24 HOURS ACROSS THESE AREAS. TROPICAL CYCLONE (TC) DEVELOPMENT IS FAVORED
ACROSS THE EAST PACIFIC BY THE BEGINNING OF WEEK-2. A NORTHWEST TRACK OF A TC
CLOSE TO THE BAJA PENINSULA COULD BRING A SURGE OF MOISTURE NORTH FROM THE GULF
OF CALIFORNIA INTO THE DESERT SOUTHWEST LATER DURING WEEK-2. THEREFORE, THE
RISK OF FLASH FLOODING MAY INCREASE ACROSS THE DESERT SOUTHWEST BY THE
BEGINNING OF SEPTEMBER.



THE U.S. DROUGHT MONITOR VALID ON AUG 15 INDICATES THAT SEVERE TO EXCEPTIONAL
(D2-D4) DROUGHT COVERAGE DECREASED SLIGHTLY FROM 5.38 PERCENT TO 5.33 PERCENT
ACROSS THE CONTINENTAL U.S.

FORECASTER: BRAD PUGH

$$




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