Prognostic Meteorological Discussion
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FXUS21 KWNC 251901
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT JULY 25 2017

SYNOPSIS: A COLD FRONT IS FORECAST TO BE ADVANCING THROUGH THE EASTERN HALF OF
THE COUNTRY AS THE FORECAST PERIOD BEGINS, AND MAY MAKE IT TO THE GULF OF
MEXICO BY THE END OF WEEK-1. ACROSS THE WEST, SURFACE HIGH PRESSURE IS FORECAST
NEAR THE FOUR CORNERS AREA THROUGHOUT THE WEEK, WHILE MID-LEVEL HIGH PRESSURE
IS ANTICIPATED TO DRIFT SLOWLY WESTWARD. MID-LEVEL HIGH PRESSURE CONTINUES TO
BE FORECAST NEAR THE PACIFIC COAST IN WEEK-2, WITH MID-LEVEL LOW PRESSURE
ACROSS THE GREAT LAKES. MID-LEVEL LOW PRESSURE IS FORECAST NEAR SOUTHWESTERN
ALASKA DURING THE FORECAST PERIOD.

HAZARDS

HEAVY RAIN ACROSS PORTIONS OF THE MID-ATLANTIC AND THE NORTHEAST, FRI, JUL 28.

HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST AND THE MID-ATLANTIC, SAT-MON, JUL
29-JUL 31.

HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN ROCKIES AND THE CENTRAL
AND SOUTHERN PLAINS, SUN-MON, JUL 30-JUL 31.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE UPPER AND MIDDLE
MISSISSIPPI VALLEY AND THE GREAT LAKES.

FLOODING LIKELY ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY.

FLOODING POSSIBLE ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS, THE
CENTRAL AND SOUTHERN ROCKIES, THE CENTRAL GREAT BASIN, AND THE SOUTHWEST.

MODERATE RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL
VALLEY OF CALIFORNIA, WED-THU, AUG 2-AUG 3.

SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PORTIONS OF THE GREAT BASIN,
THE NORTHERN PLAINS, THE NORTHERN ROCKIES, CALIFORNIA, THE PACIFIC NORTHWEST,
AND THE SOUTHWEST, WED-SUN, AUG 2-AUG 6.

SEVERE DROUGHT ACROSS PARTS OF THE GREAT PLAINS, ARIZONA, CALIFORNIA, AND
HAWAII.

DETAILED SUMMARY

FOR FRIDAY JULY 28 - TUESDAY AUGUST 01: DURING THE COURSE OF WEEK-1, INCREASING
AMPLITUDE IN THE MID-LEVEL FLOW IS ANTICIPATED ACROSS THE CONUS, WITH RIDGING
BECOMING FOCUSED ACROSS THE WESTERN U.S. AND TROUGHING FURTHER EAST. AT THE
START OF THE FORECAST PERIOD, A COLD FRONT IS FORECAST TO BE PUSHING EASTWARD
IN ADVANCE OF THE MID-LEVEL TROUGHING. INITIAL HAZARDS WITH THE COLD FRONT ARE
ANTICIPATED TO LIMITED TO HEAVY RAIN (EXCEEDING 1" IN 24 HOURS) ACROSS PORTIONS
OF THE NORTHEAST ON JULY 28. AS THE FRONT IS ANTICIPATED TO SAG SOUTHWARD, THE
HEAVY RAIN THREAT IS FORECAST TO SHIFT SOUTHWARD TO STRETCH BETWEEN THE FLORIDA
PANHANDLE AND OUTER BANKS OVER A PROLONGED PERIOD FOR JULY 29-31. BEHIND THE
FRONT, COOLER AND DRIER AIR IS FORECAST TO FILTER IN, WITH REFRESHING
BELOW-NORMAL TEMPERATURES POSSIBLE.



ACROSS THE WEST, CONDITIONS ARE ANTICIPATED TO BE LESS REFRESHING AS SURFACE
HIGH PRESSURE SETS UP ACROSS THE FOUR CORNERS REGION AND MID-LEVEL RIDGING
RETROGRADES TOWARDS THE PACIFIC COAST. NO HEAT-RELATED HAZARDS ARE ANTICIPATED
ACROSS THE WEST AT THIS TIME, AS DAILY HIGH TEMPERATURE ANOMALIES ARE GENERALLY
8 DEGREES ABOVE-NORMAL OR LESS. UPSLOPE FLOW IS FORECAST TO SETUP ACROSS
PORTIONS OF NORTH TEXAS AND INTO NEW MEXICO AND COLORADO LATE IN THE WEEK,
POTENTIALLY YIELDING HEAVY RAIN FOR THESE AREAS ON JULY 30-31. ELSEWHERE,
LOCALIZED HEAVY RAINS ARE POSSIBLE IN ASSOCIATION WITH THE NORTH AMERICAN
MONSOON, BUT THE HIGHLY LOCALIZED NATURE OF THESE SHOWERS AND THUNDERSTORMS
LIMITS ANY HAZARD FORECASTS. NEVERTHELESS, FLOODING REMAINS POSSIBLE ACROSS
MUCH OF THE SOUTHWESTERN U.S. DURING WEEK-1, WITH THE HIGHLIGHTED AREAS
APPROXIMATELY CORRESPONDING TO THE AREA RECEIVING AT LEAST 1-2" OF RAINFALL
OVER JULY 28-31 BASED ON GUIDANCE FROM THE WEATHER PREDICTION CENTER.



A COLD FRONT IS FORECAST TO DESCEND INTO NORTHERN ALASKA TOWARDS THE END OF
WEEK-1, THAT THEN BECOMES STATIONARY NEAR THE NORTH SLOPE. SURFACE HIGH
PRESSURE IS FORECAST SOUTH OF THE ALEUTIANS THROUGHOUT THE FORECAST PERIOD.
WARM CONDITIONS ARE FORECAST ACROSS THE STATE, BUT HIGH TEMPERATURES REACHING
THE 85 DEGREE F THRESHOLD FOR MUCH ABOVE-NORMAL TEMPERATURES APPEAR UNLIKELY AT
THIS TIME.



TROPICAL STORM GREG IS FORECAST TO DISSIPATE PRIOR TO THE FORECAST PERIOD, WITH
ITS REMNANTS PASSING TO THE SOUTH OF THE HAWAIIAN ISLANDS. NO IMPACTS TO THE
U.S. ARE ANTICIPATED FROM HURRICANES HILARY OR IRWIN IN THE EAST PACIFIC.

FOR WEDNESDAY AUGUST 02 - TUESDAY AUGUST 08: DURING WEEK-2, ANOMALOUS 500-HPA
RIDGING IS FORECAST IN THE VICINITY OF THE PACIFIC COASTLINE WITH THE GEFS
FURTHER NORTH AND MORE AMPLIFIED RELATIVE TO THE ECMWF ENSEMBLES. SLIGHTLY
BELOW-NORMAL HEIGHTS ARE FORECAST FROM THE GREAT LAKES THROUGH THE MISSISSIPPI
VALLEY BY THE GEFS, WHILE THE ECMWF ENSEMBLES KEEP BELOW-NORMAL HEIGHTS FOCUSED
ACROSS THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY. ANOMALOUS TROUGHING IS
ALSO FORECAST OVER THE GULF OF ALASKA DURING WEEK-1, WITH THE GEFS MORE
AMPLIFIED ONCE AGAIN RELATIVE TO THE ECMWF ENSEMBLES.



GIVEN THE RIDGING ACROSS THE WEST, HEAT-RELATED HAZARDS APPEAR POSSIBLE. THE
BEST ODDS FOR MUCH ABOVE-NORMAL TEMPERATURES CURRENTLY APPEAR TO BE FOR THE
NORTHERN CENTRAL VALLEY OF CALIFORNIA ON AUGUST 2-3. HERE, A MODERATE RISK IS
PRESCRIBED BASED ON THE GEFS PROBABILISTIC EXTREMES TOOL SUPPORTING A 40%
CHANCE OF DAILY HIGH TEMPERATURES EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE.
THIS TOOL ALSO PROVIDES AT LEAST A 20% CHANCE OF HIGH TEMPERATURES EXCEEDING
THIS SAME THRESHOLD ACROSS PORTIONS OF THE WEST AWAY FROM THE PACIFIC COAST AND
EXTENDING INTO THE NORTHERN PLAINS OVER AUGUST 2-6. GEFS REFORECAST GUIDANCE IS
UNSURPRISINGLY WARM AS WELL, ALTHOUGH ECMWF REFORECASTS ARE COOLER, IN LINE
WITH THE MORE SUPPRESSED HEIGHT ANOMALIES. EXCESSIVE HEAT GUIDANCE IN WEEK-2 IS
FOCUSED FURTHER SOUTH ACROSS SOUTHERN CALIFORNIA, HOWEVER, REFORECAST GUIDANCE
SUGGESTS THE WEAKEST PROBABILITIES FOR ABOVE-NORMAL TEMPERATURES HERE DURING
WEEK-2. GIVEN THAT THESE AREAS SEE THEIR EXCESSIVE HEAT DRIVEN NEARLY
EXCLUSIVELY BY TEMPERATURES INSTEAD OF HUMIDITY, AND THE LACK OF ANY EXCESSIVE
HEAT CONTINUING FROM WEEK-1 IN THESE AREAS, NO EXCESSIVE HEAT HAZARDS ARE
PRESENTLY FORECAST DURING WEEK-2.



THE POTENTIAL FLOODING THREAT IN ASSOCIATION WITH SHOWERS AND THUNDERSTORMS
FROM THE NORTH AMERICAN MONSOON IS ANTICIPATED TO SHIFT WESTWARD RELATIVE TO
WEEK-1. FLOODING WITH THESE LOCALIZED RAINS IS POSSIBLE DURING WEEK-2 FROM
NEVADA AND DEATH VALLEY SOUTHEASTWARD TOWARDS THE RIO GRANDE AND IS BASED ON
ENHANCED PROBABILITIES OF ABOVE-NORMAL RAINS DURING WEEK-2 FROM GEFS AND ECMWF
ENSEMBLE REFORECAST GUIDANCE.



THE U.S. DROUGHT MONITOR VALID ON JULY 18 INDICATES THAT SEVERE TO EXCEPTIONAL
(D2-D4) DROUGHT COVERAGE INCREASED TO 4.77 PERCENT ACROSS THE CONTINENTAL U.S.
THIS INCREASE IS RELATED TO AN EXPANSION OF SEVERE DROUGHT ACROSS THE NORTHERN
GREAT PLAINS.

FORECASTER: CPC FORECASTER

$$




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