Prognostic Meteorological Discussion
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FXUS21 KWNC 261952
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT APRIL 26 2017

SYNOPSIS: MID-LEVEL LOW PRESSURE IS ANTICIPATED ACROSS THE CENTRAL CONTIGUOUS
U.S. DURING MOST OF THE 3-7 DAY PERIOD, WITH A FORECAST TENDENCY TO LIFT
NORTHWARD, TOWARD AND OVER THE GREAT LAKES REGION. SURFACE LOW PRESSURE IS
FORECAST TO DEVELOP IN THE LEE OF THE SOUTHERN ROCKIES, AND INTENSIFY WHILE
SIMILARLY LIFTING NORTHWARD TOWARD THE GREAT LAKES. A WIDE ARRAY OF HAZARDS IS
POSSIBLE ASSOCIATED WITH THIS FORECAST SCENARIO, GIVEN THE TRANSITION SEASON
TIMING AND ANTICIPATED STRONG DISTURBANCE THAT IS EXPECTED TO BE IN A FAVORABLE
LOCATION TO TAP INTO GULF MOISTURE. UNCERTAINTY IS HIGH REGARDING THE WEEK-2
CIRCULATION AND ITS IMPACTS TO THE LOWER-48 STATES. ALASKA IS EXPECTED TO
EXPERIENCE PERIODIC SURFACE LOW PRESSURE SYSTEMS OF MODEST INTENSITY IN THE
BERING SEA AND GULF OF ALASKA THROUGHOUT THE OUTLOOK PERIOD.

HAZARDS

HEAVY PRECIPITATION ACROSS PORTIONS OF THE SOUTHERN ROCKIES, THE CENTRAL
ROCKIES, THE SOUTHERN PLAINS, AND THE CENTRAL PLAINS, SAT-SUN, APR 29-APR 30.

HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE LOWER MISSISSIPPI VALLEY,
THE NORTHERN PLAINS, THE TENNESSEE VALLEY, THE GREAT LAKES, THE MIDDLE
MISSISSIPPI VALLEY, THE MID-ATLANTIC, THE SOUTHERN APPALACHIANS, THE UPPER
MISSISSIPPI VALLEY, THE SOUTHEAST, THE SOUTHERN PLAINS, AND THE OHIO VALLEY,
SAT-SUN, APR 29-APR 30.

HEAVY RAIN ACROSS PORTIONS OF THE NORTHEAST, MON-TUE, MAY 1-MAY 2.

SEVERE WEATHER ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND THE SOUTHERN
PLAINS, SAT, APR 29.

SEVERE WEATHER ACROSS PORTIONS OF THE SOUTHEAST, THE MIDDLE MISSISSIPPI VALLEY,
THE LOWER MISSISSIPPI VALLEY, THE OHIO VALLEY, AND THE TENNESSEE VALLEY, SUN,
APR 30.

FLOODING POSSIBLE ACROSS PORTIONS OF THE CENTRAL PLAINS, THE LOWER MISSISSIPPI
VALLEY, THE SOUTHERN PLAINS, THE MIDDLE MISSISSIPPI VALLEY, THE GREAT LAKES,
AND THE OHIO VALLEY.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE MID-ATLANTIC, THE OHIO
VALLEY, THE NORTHERN PLAINS, THE NORTHERN GREAT BASIN, AND THE TENNESSEE VALLEY.

FLOODING LIKELY ACROSS PORTIONS OF THE SOUTHEAST AND THE MID-ATLANTIC.

EXCESSIVE HEAT ACROSS PORTIONS OF THE SOUTHERN PLAINS, SAT, APR 29.

HIGH WINDS ACROSS PORTIONS OF THE SOUTHERN ROCKIES, THE SOUTHERN PLAINS,
CALIFORNIA, THE CENTRAL PLAINS, AND THE SOUTHWEST, SAT, APR 29.

HIGH WINDS ACROSS PORTIONS OF THE CENTRAL PLAINS, THE LOWER MISSISSIPPI VALLEY,
THE NORTHERN PLAINS, THE GREAT LAKES, THE MIDDLE MISSISSIPPI VALLEY, THE UPPER
MISSISSIPPI VALLEY, THE SOUTHERN PLAINS, AND THE OHIO VALLEY, SUN-MON, APR
30-MAY 1.

MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY,
THE CENTRAL APPALACHIANS, THE TENNESSEE VALLEY, THE MID-ATLANTIC, THE SOUTHERN
APPALACHIANS, THE SOUTHEAST, THE GREAT LAKES, AND THE OHIO VALLEY, SAT-SUN, APR
29-APR 30.

MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE CENTRAL PLAINS, THE
CENTRAL ROCKIES, THE CENTRAL GREAT BASIN, THE SOUTHERN ROCKIES, THE SOUTHERN
PLAINS, AND THE SOUTHWEST, SUN, APR 30.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL
PLAINS, THE CENTRAL ROCKIES, THE CENTRAL GREAT BASIN, THE NORTHERN PLAINS, THE
NORTHERN ROCKIES, THE SOUTHERN ROCKIES, THE UPPER MISSISSIPPI VALLEY, THE GREAT
LAKES, AND THE SOUTHWEST, THU-SAT, MAY 4-MAY 6.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL GREAT
BASIN, CALIFORNIA, AND THE SOUTHWEST, THU-SAT, MAY 4-MAY 6.

SEVERE DROUGHT ACROSS THE LOWER MISSISSIPPI VALLEY, THE TENNESSEE VALLEY,
HAWAII, THE NORTHEAST, CALIFORNIA, THE SOUTHEAST, THE SOUTHERN APPALACHIANS,
THE SOUTHERN PLAINS, AND THE SOUTHWEST.

DETAILED SUMMARY

FOR SATURDAY APRIL 29 - WEDNESDAY MAY 03: THE PRIMARY FEATURE OF INTEREST
DURING THE 3-7 DAY PERIOD IS AN ANTICIPATED 500-HPA TROUGH STRETCHING FROM
SOUTHERN CANADA THROUGH NORTHERN MEXICO ACROSS THE CENTRAL CONUS. EARLY IN THIS
PERIOD THE TROUGH IS FORECAST TO AMPLIFY OVER THE ROCKIES, BEFORE BECOMING
NEGATIVELY TILTED AND LESSENING IN AMPLITUDE AS IT LIFTS TOWARD THE GREAT LAKES
THROUGH EARLY NEXT WEEK. LEE SURFACE CYCLOGENESIS IS LIKELY OVER THE SOUTHERN
ROCKIES ASSOCIATED WITH THE MID-LEVEL TROUGH, WITH THIS SYSTEM BECOMING THE
PRIMARY DRIVER OF HAZARDOUS WEATHER THROUGHOUT THE 3-7 DAY FORECAST PERIOD.



DRY, DOWNSLOPE WESTERLY WINDS ARE FORECAST AROUND THE BASE OF THE MID-LEVEL
TROUGH ACROSS THE SOUTHERN HIGH PLAINS AS THE PERIOD BEGINS AND THE SURFACE
CYCLONE BEGINS TO DEVELOP. WITH A MORE SOUTHERLY COMPONENT TO THE FLOW FARTHER
EAST, A DRYLINE IS ANTICIPATED TO DEVELOP ACROSS TEXAS WHILE A WARM FRONT
EXTENDS EAST OF THE SURFACE LOW THROUGH THE OZARKS. THE STORM PREDICTION CENTER
(SPC) PREDICTS A 30% CHANCE OF SEVERE WEATHER FOR A FAIRLY BROAD AREA CENTERED
NEAR THE ARKLATEX ASSOCIATED WITH THESE FEATURES, PRIMARILY NEAR THE TRIPLE
POINT AND ALONG THE WARM FRONT ON SAT, APR 29, WHILE A 15% RISK IS PREDICTED
THE FOLLOWING DAY ACROSS PORTIONS OF THE LOWER MISSISSIPPI AND LOWER TENNESSEE
VALLEYS, PRIMARILY DUE TO THE APPROACHING COLD FRONT. AMPLE MOISTURE AND
INSTABILITY APPEAR TO BE AVAILABLE WITH THE FLOW OF WARM, GULF AIR NORTHWARD,
WHILE WIND SHEAR SHOULD ALSO BE SUBSTANTIAL GIVEN THE DEEPENING SURFACE LOW AND
INTENSIFYING LOW-LEVEL JET, COUPLED WITH SOUTHWESTERLY FLOW ALOFT. THIS
SCENARIO SUGGESTS SEVERE THUNDERSTORM DEVELOPMENT WITH A RISK OF LARGE HAIL AND
TORNADOES. ASIDE FROM THE SEVERE WEATHER, WIDESPREAD HEAVY RAIN (EXCEEDING 1"
IN 24 HOURS) IS FORECAST ON APR 29-30, ACROSS A LARGE PORTION OF THE
EAST-CENTRAL CONUS. OVER THE COURSE OF THIS (MOSTLY) TWO-DAY EVENT, MUCH OF
THIS SAME REGION IS FORECAST TO RECEIVE AS MUCH AS 3-6" OF RAIN. LATER IN THE
PERIOD, FROM MAY 1-2, HEAVY RAIN (1-2") IS EXPECTED OVER MAINE, AS THE FRONTAL
SYSTEM APPROACHES AND TRAVERSES THAT AREA.



ON THE WEST SIDE OF THE SURFACE LOW PRESSURE, HEAVY PRECIPITATION (1" OR
GREATER LIQUID EQUIVALENT IN 24 HOURS) IS FORECAST OVER SOUTHEASTERN COLORADO
AND SMALL PARTS OF ADJACENT STATES ON APRIL 29-30. SNOW IS POSSIBLE ACROSS THE
HIGHER ELEVATIONS OF THE FRONT AND PARK RANGES, WITH DAILY TOTALS OF PERHAPS
6", WHILE RAIN IS FORECAST AT LOWER ELEVATIONS. SOME SNOW MAY MAKE IT INTO THE
IMMEDIATE DENVER METRO AREA GIVEN THE STRONG COLD AIR ADVECTION ON THE WEST
SIDE OF THE LOW AND SUBSTANTIAL COLD AIR ALOFT. MUCH BELOW-NORMAL TEMPERATURES
ARE FORECAST FOR A SIMILAR, THOUGH SOMEWHAT LARGER, REGION ON APR 30, WHERE
MINIMUM TEMPERATURE ANOMALIES OF -12 TO -16 DEGREES F OR MORE ARE POSSIBLE THAT
COULD RESULT IN A HARD FREEZE FOR MANY AREAS. WIDESPREAD HIGH WINDS (EXCEEDING
30 KTS) ARE ALSO POSSIBLE ON THE BACK SIDE OF THE SURFACE LOW PRESSURE AS IT
DEEPENS AND LIFTS NORTHWARD WITH AN ASSOCIATED HAZARD OVER THE SOUTHERN HIGH
PLAINS ON APR 29, AND FROM SOUTH-CENTRAL PORTIONS OF BOTH THE GREAT PLAINS AND
MISSISSIPPI VALLEY TO THE GREAT LAKES REGION ON APR 30-MAY 1.



MARGINALLY ELEVATED FIRE WEATHER CONDITIONS ARE POSSIBLE ALONG THE FAR SOUTHERN
CALIFORNIA COAST ON APR 29, WHERE DRY, FAIRLY STRONG, DOWNSLOPE WINDS ARE
ANTICIPATED WITH ANTECEDENT DRY CONDITIONS. IN ADDITION, TEMPERATURES IN THE
LOS ANGELES AREA MAY REACH 90 DEGREES F ON THE SAME DAY, BUT DUE TO THE HEAT
BEING SHORT-LIVED AND ON A VERY SMALL SCALE, NO HEAT HAZARD WAS INTRODUCED ON
THE MAP FOR THIS AREA.



AHEAD OF THE AFOREMENTIONED LOW PRESSURE SYSTEM, WIDESPREAD WARM ADVECTION IS
FORECAST AND ASSOCIATED HEAT-RELATED HAZARDS ARE POSSIBLE BEFORE RELIEF ARRIVES
FROM THE COLD FRONTAL PASSAGE. FIRST, EXCESSIVE HEAT IS FORECAST FOR PORTIONS
OF SOUTH TEXAS ON APR 29, AS HEAT INDEX VALUES MAY EXCEED 105 DEGREES F
ASSOCIATED WITH THE WARM, HUMID CONDITIONS. MAXIMUM TEMPERATURES ARE EXPECTED
TO RANGE BETWEEN 12-20 DEGREES F ABOVE-NORMAL ACROSS THE UPPER OHIO VALLEY,
TENNESSEE VALLEY, SOUTHEAST CONUS, MUCH OF THE MID-ATLANTIC, AND THE SOUTHERN
AND CENTRAL APPALACHIANS FROM APR 29-30. MANY AREAS MAY APPROACH OR EXCEED 90
DEGREES F FOR THE FIRST TIME THIS YEAR.



FLOODING IS POSSIBLE, LIKELY, IMMINENT, OR OCCURRING ACROSS THE CAROLINAS AND
SOUTHERN VIRGINIA, THE MIDWEST, IN THE GENERAL VICINITY OF EASTERN OKLAHOMA AND
WESTERN AND CENTRAL ARKANSAS, NORTHERN NORTH DAKOTA, AND PARTS OF IDAHO. A COLD
FRONT WHICH STALLED ACROSS NORTH CAROLINA AND TENNESSEE THIS PAST WEEK BROUGHT
SEVERE STORMS TO THE REGION, RAINFALL AMOUNTS OF 2-10", AND FLASH FLOODING.



ALASKA IS FORECAST TO EXPERIENCE RELATIVELY QUIET WEATHER DURING DAYS 3-7, WITH
DISTURBANCES OF MODEST TO MODERATE INTENSITY, BY ALASKAN STANDARDS, OVER THE
GULF OF ALASKA AND BERING SEA. UNLIKE THE CONTIGUOUS U.S., NO SUBSTANTIAL
TEMPERATURE ANOMALIES ARE FORECAST, GIVEN THE LACK OF ANOMALOUS CIRCULATIONS
FORECAST IN THE VICINITY OF THE STATE.

FOR THURSDAY MAY 04 - WEDNESDAY MAY 10: SUBSTANTIAL UNCERTAINTY IS APPARENT IN
THE WIDE RANGE OF SPAGHETTI MAP PERSPECTIVES OF THE MID-LEVEL CIRCULATION OVER
NORTH AMERICA IN WEEK-2. THERE IS A SLIGHT TENDENCY TOWARD ANOMALOUS 500-HPA
RIDGING OVER THE WEST COAST, WITH NEGATIVE HEIGHT ANOMALIES CENTERED ON THE
GREAT LAKES. GIVEN THE REDUCED AMPLITUDE OF THE CIRCULATION ANTICIPATED DURING
WEEK-2, LIMITED HAZARDOUS IMPACTS CAN BE DISCERNED AT THIS TIME.



A SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES IS FORECAST FOR PORTIONS OF THE
SOUTHWEST AND CALIFORNIA FOR AT LEAST THE FIRST FEW DAYS OF THE WEEK-2 PERIOD,
WHERE RE-FORECAST GUIDANCE IS CONSISTENTLY WARM BENEATH THE FORECAST ANOMALOUS
RIDGING. THE AREA INDICATED ON THE MAP IS A CONFLUENCE OF REGIONS FORECAST BY
THE GEFS TO HAVE AT LEAST A 20% CHANCE OF BOTH EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE OF MAXIMUM TEMPERATURE AND REACHING 85 DEGREES F. A
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES EXTENDS FROM THE CENTRAL ROCKIES
THROUGH THE HIGH PLAINS AND EASTWARD THROUGH THE GREAT LAKES FOR THU-SAT, MAY
4-6. HERE LOW TEMPERATURES MAY FALL BELOW FREEZING, WHILE THE GEFS SUPPORTS AT
LEAST A 20% CHANCE OF BEING BELOW THE 15TH CLIMATOLOGICAL PERCENTILE OF MINIMUM
TEMPERATURE. THE BEST CHANCE OF MUCH ABOVE-NORMAL TEMPERATURE ANOMALIES APPEARS
TO BE IN ALASKA DURING WEEK-2, WITH ANOMALOUS SOUTHERLY FLOW ANTICIPATED, BUT
NO HAZARDS ARE FORECAST GIVEN THE MODEST HIGH TEMPERATURES AT THIS TIME OF YEAR
AND FIRE SEASON REMAINING WELL INTO THE FUTURE.



ACCORDING TO THE U.S. DROUGHT MONITOR RELEASED ON APRIL 20TH (USING DATA
THROUGH 8 AM EDT, APRIL 18TH), COVERAGE OF SEVERE, OR GREATER INTENSITY,
DROUGHT THROUGHOUT THE CONUS REMAINS THE SAME AS LAST WEEK (1.44 PERCENT). THIS
REMAINS THE LOWEST COVERAGE OF D2-D4 DROUGHT OVER THE CONUS SINCE AUG 2010.

FORECASTER: ANTHONY ARTUSA

$$



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