Prognostic Meteorological Discussion
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FXUS21 KWNC 261756
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT APRIL 26 2018

SYNOPSIS: A MID-LEVEL LOW PRESSURE SYSTEM IS FORECAST TO SLOWLY PROGRESS EAST
FROM THE SOUTHWEST TO THE CENTRAL U.S. EARLY IN THE PERIOD, WHILE MID-LEVEL
HIGH PRESSURE BRIEFLY BUILDS OVER THE EASTERN U.S. EARLY IN WEEK-2, MID-LEVEL
LOW (HIGH) PRESSURE IS LIKELY TO DEVELOP OVER EASTERN (WESTERN) NORTH AMERICA.
MULTIPLE SURFACE LOWS ARE FORECAST TO TRACK ACROSS THE BERING SEA AND ALEUTIANS
DURING THE NEXT TWO WEEKS.

HAZARDS

HEAVY PRECIPITATION (RAIN AND HIGH-ELEVATION SNOW) FOR THE NORTHERN ROCKIES,
SUN-MON, APR 29-30.

SEVERE WEATHER FOR THE CENTRAL AND SOUTHERN PLAINS AND PARTS OF THE MIDDLE
MISSISSIPPI VALLEY, TUE-WED, MAY 1-2.

HEAVY RAIN SHIFTING EAST FROM THE CENTRAL AND SOUTHERN PLAINS TO THE LOWER AND
MIDDLE MISSISSIPPI VALLEY, TUE-THU, MAY 1-3.

HIGH WINDS FOR THE SOUTHWEST AND CENTRAL/SOUTHERN HIGH PLAINS, MON-WED, APR
30-MAY 2.

A SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE LOWER MISSISSIPPI VALLEY
AND SOUTHEAST, FRI, MAY 4.

A SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR THE EASTERN CORN BELT,
GREAT LAKES, CENTRAL APPALACHIANS, AND PARTS OF THE NORTHEAST, FRI-MON, MAY 4-7.

A SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR THE DESERT SOUTHWEST,
FRI-THU, MAY 4-10.

FLOODING OCCURRING, IMMINENT, OR LIKELY ACROSS PARTS OF THE UPPER MISSISSIPPI
VALLEY, MISSOURI RIVER VALLEY, AND RED RIVER OF THE NORTH.

FLOODING POSSIBLE ACROSS PARTS OF THE NORTHERN ROCKIES.

SEVERE DROUGHT ACROSS PARTS OF THE SOUTHEAST, CENTRAL AND SOUTHERN PLAINS,
SOUTHWEST, AND CALIFORNIA.

DETAILED SUMMARY

FOR SUNDAY APRIL 29 - THURSDAY MAY 03: AN AMPLIFYING LONGWAVE TROUGH AND ITS
SLOW EASTWARD PROGRESSION FROM THE SOUTHWESTERN TO THE CENTRAL U.S. IS FORECAST
TO RESULT IN A MULTI-DAY RISK OF SEVERE THUNDERSTORMS, HEAVY RAIN, AND FLASH
FLOODING ACROSS THE CENTRAL/SOUTHERN PLAINS AND MIDDLE MISSISSIPPI VALLEY
DURING THIS PERIOD. AS SURFACE HIGH PRESSURE SHIFTS OFFSHORE OF THE EAST COAST
EARLY NEXT WEEK, LOW-LEVEL MOISTURE FROM THE WESTERN GULF IS LIKELY TO INCREASE
ACROSS THE SOUTH-CENTRAL U.S. SEVERE THUNDERSTORMS, INCLUDING TORNADIC
SUPERCELLS, ARE EXPECTED TO INITIATE ALONG A DRYLINE ACROSS THE CENTRAL AND
SOUTHERN PLAINS ON MAY 1 AND 2. THE EVOLVING PATTERN FAVORS A WIDESPREAD AREA
OF HEAVY RAINFALL (2 TO 5 INCHES) ACROSS THE CENTRAL/SOUTHERN PLAINS EAST TO
THE LOWER/MIDDLE MISSISSIPPI VALLEY FROM MAY 1 TO 3.



THE DETERMINISTIC MODEL SOLUTIONS INDICATE A LEE SIDE LOW AND ASSOCIATED
DRYLINE REMAINING OVER THE CENTRAL AND SOUTHERN HIGH PLAINS FROM APRIL 30 TO
MAY 2. BEHIND THIS DRYLINE, GUSTY WINDS (ABOVE 40MPH) ARE FORECAST TO AFFECT
THE SOUTHWESTERN U.S. THESE GUSTY WINDS ALONG WITH VERY LOW RELATIVE HUMIDITY
VALUES ARE LIKELY TO MAINTAIN ELEVATED FIRE WEATHER CONCERNS AND OCCASIONAL
BLOWING DUST EARLY NEXT WEEK.



DOWNSTREAM OF THE AMPLIFIED TROUGH, 500-HPA HEIGHTS ARE FORECAST TO INCREASE TO
588DM OVER THE SOUTHEASTERN U.S., SUPPORTING AT LEAST A BRIEF PERIOD OF
ABOVE-NORMAL TEMPERATURES ALONG THE EAST COAST. ALTHOUGH MAXIMUM TEMPERATURES
ARE EXPECTED TO AVERAGE 10 TO 20 DEGREES F ABOVE NORMAL ACROSS THE MID-ATLANTIC
AND NORTHEAST, BEGINNING ON MAY 1, MAXIMUM TEMPERATURES IN THE 80S ARE NOT
CONSIDERED HAZARDOUS. BASED ON 850-HPA TEMPERATURES AT OR ABOVE 15 DEGREES F
ALONG WITH DOWNSLOPE FLOW, MAXIMUM TEMPERATURES COULD APPROACH 90 DEGREES F
ACROSS THE MID-ATLANTIC ON MAY 2 AND 3. PRECEDING THIS WARMUP, PATCHY FROST IS
EXPECTED AS FAR SOUTH AS THE EASTERN CORN BELT AND CENTRAL APPALACHIANS ON
MONDAY MORNING.



ON APRIL 29 AND 30, A CLOSED LOW AT 500-HPA IS FORECAST TO ADVANCE EAST FROM
THE PACIFIC NORTHWEST TO THE NORTHERN ROCKIES. HEAVY SNOW (TOTAL AMOUNTS OF 6
TO 12 INCHES) IS EXPECTED ACROSS THE HIGHER ELEVATIONS OF THE NORTHERN ROCKIES,
WHILE HEAVY RAIN (NEAR AN INCH) MAY WORSEN ANY ONGOING SMALL STREAM AND RIVER
FLOODING ACROSS THIS REGION.



MINOR TO MODERATE FLOODING CONTINUES ALONG THE LOWER MISSISSIPPI RIVER. HEAVY
RAINFALL AT THE BEGINNING OF MAY IS EXPECTED TO PROLONG THE FLOODING. SMALL
STREAM AND RIVER FLOODING, ASSOCIATED WITH SNOW MELT, IS OCCURRING ACROSS THE
UPPER MISSISSIPPI VALLEY, MISSOURI RIVER VALLEY, AND THE NORTHERN ROCKIES.
FLOODING HAS REACHED MODERATE LEVELS ALONG PARTS OF THE RED RIVER OF THE NORTH.

FOR FRIDAY MAY 04 - THURSDAY MAY 10: A SLIGHT RISK OF HEAVY PRECIPITATION IS
POSTED FOR PARTS OF THE WESTERN GULF COAST, LOWER MISSISSIPPI VALLEY AND
SOUTHEAST WHERE HEAVY RAIN IS MOST LIKELY TO LINGER INTO THE WEEK-2 PERIOD. THE
DETERMINISTIC 0Z ECMWF MODEL INDICATES 1 TO 3 INCHES ACROSS SOUTHEAST TEXAS AND
SOUTHERN LOUISIANA ON MAY 4. ONCE THE TROUGH AXIS SHIFTS EAST, A MUCH DRIER
PATTERN WITH ANOMALOUS NORTHERLY FLOW IS EXPECTED ACROSS THE CENTRAL U.S.
DURING THE REMAINDER OF WEEK-2.



THE GFS AND ECMWF ENSEMBLE MEANS CONTINUE TO CONVERGE ON HOW THE LONGWAVE
PATTERN EVOLVES EARLY IN WEEK-2. THIS LONGWAVE PATTERN FEATURES TROUGHS
CENTERED ACROSS THE NORTH PACIFIC AND EASTERN NORTH AMERICA WITH ANOMALOUS
RIDING OVER WESTERN NORTH AMERICA. THE LOCATION AND AMPLITUDE OF THE LONGWAVE
FEATURES ARE VERY SIMILAR BETWEEN THE GFS AND ECMWF ENSEMBLE MEANS BY DAY 9. A
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES IS POSTED FOR THE GREAT LAKES,
EASTERN CORN BELT, CENTRAL APPALACHIANS, AND PARTS OF THE NORTHEAST WHERE A
FROST AND/OR LIGHT FREEZE MAY OCCUR. ALTHOUGH THE GROWING SEASON IS OFF TO A
SLOW START IN THESE AREAS, A LIGHT FREEZE MAY DAMAGE EARLY VEGETATIVE GROWTH.
THE 6Z GEFS INDICATES DAILY TEMPERATURES AVERAGING MORE THAN 8 DEGREES F ABOVE
NORMAL THROUGHOUT MUCH OF THE WEEK-2 PERIOD ACROSS THE SOUTHWESTERN U.S.,
SUPPORTING A SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES. ABOVE-NORMAL
TEMPERATURES ARE LIKELY TO CAUSE RAPID SNOW MELT AND HIGH FLOWS ALONG STREAMS
AND RIVERS THROUGHOUT THE NORTHERN ROCKIES AND UPPER MISSOURI RIVER VALLEY
THROUGH EARLY MAY.



THE US DROUGHT MONITOR, VALID ON APRIL 24, INDICATES THAT THE COVERAGE OF
SEVERE TO EXCEPTIONAL DROUGHT REMAINED NEARLY STEADY AT 16.35 PERCENT. DROUGHT
CONTINUES TO INTENSIFY ACROSS THE SOUTHWEST, WHILE DROUGHT AMELIORATION
CONTINUES ACROSS MUCH OF THE SOUTHEAST WITH THE FREQUENT RAINFALL SINCE EARLY
APRIL.

FORECASTER: BRAD PUGH

$$




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