Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Thu May 17 2018
Valid 12Z Sun May 20 2018 - 12Z Thu May 24 2018
16 UTC Update...
Model guidance has started to stabilize with the timing of an
impulse of shortwave energy ejecting out of the central
Rockies/Plains on Sunday, although there is still some uncertainty
with the details of an associated weak surface wave, especially as
it lifts out of the middle Mississippi Valley and towards the
lower Great Lakes and Ohio Valley Sunday night. In regards to the
upper low expected to amplify over the western U.S. early next
week, the majority of 00Z model guidance shifted towards a weaker
low closing off near California on Monday, and the updated WPC
medium range forecast reflects this weaker shift in the guidance.
The updated WPC medium range forecast stayed fairly close to
ensemble means and continuity late in the forecast period in order
to account for the wide range of solutions and large run-to-run
variability with the evolution of flow over the eastern Pacific
during the later half of next week.
Gerhardt
...Guidance and Predictability Assessment...
A model and ensemble composite seems to provide a reasonable
mid-larger scale pattern evolution over medium range time scales.
However, the focus of embedded mid-smaller scale systems still
offers quite a bit of timing/emphasis variance, especially over
the n-central U.S. Latest model trends there support less
progressive srn stream impulse/surface wave moisture and
instability focus in a wetter convective pattern threat now more
separated from nrn stream influence early next week. The 12/00 UTC
UKMET seem to best support this. The 00 UTC ECMWF drastically
slowed compared to its prior 12 UTC run and recent GFS runs have
been slower.
...Weather/Threats Highlights...
An amplifying ern Pacific trough should dig into CA and the Great
Basin next week to form a closed low aloft to support modest
widespread precipitation mainly along/north of the system.
Downstream, srn stream shortwave trough energy is now expected to
slowly eject northeastward from the Rockies to the N-Central U.S.
A wavy frontal boundary will focus anomalously deepened pooled
moisture/instabilty to fuel some heavier local convection on the
leading edge of cooling post-frontal high pressure spilling
southward over the Plains. A wavy frontal zone will meanwhile
persist and be periodically reinforced 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 near
the front supports showers and storms with a threat of local
downpours.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
Schichtel