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Prognostic Meteorological Discussion (PMD)

Issued by National Weather Service (NWS)

FXUS21 KWNC 221848
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 22 2019

SYNOPSIS: Week-2 is expected to begin with upper-level low pressure over the 
western U.S. and high pressure over the east. This pattern is forecast to break 
down pretty early, with the low pressure lifting northeastward over the Great 
Lakes and into Canada, the high pressure overspreading the much of the eastern 
and central U.S. and zonal flow establishing for the latter half of the week.  
Excessive heat is a concern for the Southeast which is likely to start as a dry 
heat, with heat index taking more of a role as dew points increase mid-period.


Moderate risk of heavy precipitation for portions of the Central Plains and 
Middle Mississippi Valley, Thu-Sun, May 30-Jun 2.

Slight risk of heavy precipitation for portions of the Central Plains and 
Mississippi Valley, Thu-Mon, May 30-Jun 3.

High risk of excessive heat for portions of Alabama, Georgia and South 
Carolina, Thu-Sun, May 30-Jun 2.

Moderate risk of excessive heat for portions of the Southeast, Thu-Mon, May 
30-Jun 3.

Slight risk of excessive heat for portions of the Lower Mississippi Valley, the 
Tennessee Valley, and the Southeast, Thu-Wed, May 30-Jun 5.



FOR THURSDAY MAY 30 - WEDNESDAY JUNE 05: Model differences give some 
uncertainty to the 500hPa height pattern that will dictate the week-2 forecast. 
The GEFS has further deamplified the mean ridge over the Southeast for days 
8-14, while the ECMWF has shifted the troughing over the Great Lakes northward 
over Canada, allowing the positive 500hPa height anomalies over the Southeast 
to spread into the Tennessee Valley. As this ridge starts to deamplify 
mid-week, the ridge remains over the Gulf Coast in the ECMWF, while the GEFS 
pushes the ridge further west over Texas. Both indicate more zonal flow for the 
latter half of the period. This ridge placement has caused differences in the 
forecast for heat in the Southeast from the two models. The ECMWF solution has 
a much longer heat event, and as zonal flow takes over mid-week, dew points 
start to rise and play more of a role in the risk for excessive heat.  Enhanced 
moisture influx from the Gulf of Mexico in connection with a possible tropical 
disturbance also will aid in increasing dew point temperatures. The GEFS is not 
quite as warm as the ECMWF but does indicate a possible second spike in heat 
anomalies mid-week from the probabilistic tool. Due to the consistency over the 
past few runs of this ECMWF solution, it is favored. The high risk for 
excessive heat over southern Alabama, Georgia and South Carolina has been 
extended from yesterday's forecast, valid from May 30-Jun 2, because of the 
influence humidity is likely to start having mid-period. A broader moderate 
risk is forecast until June 3rd through the Deep South, while a slight risk, 
through June 5rd, is extended through parts of the Mid-Atlantic and central 
Gulf Coast. With the lack of precipitation over the next two weeks and the 
duration of this heat event, there is a possibility of flash drought 
development in parts of the region, especially in the high and moderate risk 

This classic ridge-trough pattern over the CONUS is likely to set up a mean 
baroclinic zone aligning from the central and southern Plains prior to the 
period and for the few first days of week-2. The upper-level pattern is 
expected to be zonal over the central U.S. mid-week, but surface lows are 
forecast over the High Plains and drop over the central and southern Plains 
into the middle Mississippi Valley. Both the ECMWF and GEFS probabilistic tools 
indicate a 20% chance of 2 day rainfall totals reaching 1 inch through day 12. 
The ECMWF shows a much stronger signal for an event mid-period (40-60% chance 
of surpassing 85th climatological percentile), while the GEFS is weaker (20%).  
A moderate risk of heavy precipitation is posted over the region likely to be 
most impacted by these surface systems from northeast Texas northward toward 
eastern Missouri for May 30-June 2. Much of the region highlighted for both the 
slight and moderate precipitation risks are currently at 95-99 percentile for 
soil moisture with the anomalously high amount of rainfall they have received 
this spring. River and flash flooding will likely be easily triggered due to 
current conditions.

Upper-level ridging over mainland Alaska is forecast to persist through most of 
the period, though fairly weak. An upper-level trough over the Aleutians will 
likely lead to fairly active weather throughout the Gulf of Alaska, there are 
no hazards forecast for the state.

FORECASTER: Christina Maurin