WPC Medium Range Archive
Medium Range Products Valid On:
(Day 7 PMDEPD: Valid 05/18/2018 to 05/22/2018)
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
953 AM EDT Tue May 15 2018
Valid 12Z Fri May 18 2018 - 12Z Tue May 22 2018
...Cluster of showers/storms near fronts in the central Plains/
mid MS Valley and northern mid Atlantic to southern New England...
...Guidance, Predictability Assessment, and Preferences...
The models show a deep layer over the Great Basin ejecting to the
north across the Pacific northwest through Sat and becoming
ill-defined on Sunday. Heights increase in response to the next
upper trough moving onshore from the Eastern Pacific into CA. The
models have trended a little slower and deeper with the upper
trough, with the 06z GFS showing the strongest upper level low.
Teleconnections showing a modest upper trough downstream from the
closed low south of AK. With better clustering of the 00z GFS with
the respective ensemble means of the 00z GEFS and 00z ECMWF
Ensemble Means, the 06z GFS was not used in favor of the 00z run.
The 00z ECMWF was on the southern edge of the cluster of solutions
and thus the mean was given more weighting Mon and Tue 21-22 May,
especially considering better than average agreement among the
The 06z run of the GFS also is more amplified with the Great Lakes
to northeast trough next Mon-Tue 22 May. Given higher heights in
the southeast and off the coast with the persistent anticyclone in
place to abut trough development, the 06z GFS looks to be
The days 3-7 height/sea level pressure forecasts were derived from
a blend of the 00z GFS/ECMWF/00z GEFS Mean/00z ECMWF Ensemble Mean
days 3 through 5 with more weighting on the 00z GEFS/ECMWF
ensemble means days 6-7.
A few waves of lower heights/pressures emerge from the Rockies
across the high Plains of southeast WY and northeast CO across the
central Plains, with several models showing locally heavy rain in
the central high Plains across the rest of the central Plains Fri
into Sat 19 May.
Anomalously deep moisture from the Gulf of Mexico and western
Atlantic will likely spread showers and storms from the Southeast
to up the Appalachians and Mid-Atlantic/Northeast later week into
the weekend. The models forecast widespread showers when the
moisture intercepts the front across Long Island and adjacent
southern New England/nearby islands Fri into early this weekend.
In this pattern, late in the week record high temps are expected
in parts of Texas to the mid-lower MS valley before the airmass
modifies and cools early next week. Record warm mins are possible
for the Mid-Atlantic in the warm, humid airmass.
An amplified mid-upper level trough and frontal system slowly work
north over the Great Basin Fri and Rockies through the weekend,
with showers developing in areas of combined terrain lift and
deformation. The next upper trough coming onshore early next week
produces showers in CA/NV/OR/ID.
A persistent and wavy frontal zone extends east from the Plains to
the MS Valley, Oh Valley, and Northeast. Hot and moist conditions
persist to the south of the front. Instability and moisture
pooling along the front will support showers and storms with
potential for locally heavy downpours.
Other showers/storms develop in the TX/OK panhandles near the
dryline and return low level southerly flow advects moisture into
the region. Also, FL has diurnal showers/storms developing with
the GEFS Mean showing 2 inches of precipitable water, supporting
locally heavy downpours where sea/lake breeze convergence occurs.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:
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