Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 PM EDT Sun Apr 11 2021
Valid 12Z Wed Apr 14 2021 - 12Z Sun Apr 18 2021
...North-Central Rockies/High Plains Heavy Snow Threat Midweek...
...Overview and Guidance/Predictability Assessment...
The medium range period begins Wednesday with a deep closed low
over the Great Lakes, which pushes a cold front/deepening surface
low off the East Coast by Thursday/Friday. Models/ensembles
(especially the 00z ECMWF) are trending towards a more
amplified/less progressive solution, and show relatively good
agreement through much of the period with this feature. Meanwhile,
out West, a closed low will settle over the Great Basin region
during the middle of the week before a building ridge over the
West Coast/western Canada acts to push the energy eastward into
the Plains by next weekend. The models show good agreement the
first half of the period, but begin to diverge with the details of
energy ejection days 5-7. A solution close to that of the ensemble
means should help to mitigate these detail differences.
This cycle of the WPC medium range product suite was primarily
derived from a majority blend of the deterministic models
(GFS/ECMWF/CMC) through day 5, quickly increasing the weighting of
the ensemble means thereafter. This maintains overall good WPC
continuity, with one exception being a slower/farther west cold
front progression off the East Coast days 3-4, consistent with the
latest model trends.
...Weather/Hazards Highlights...
Combination closed low held over the West and lead cold Canadian
High Pressure (with temp anomalies ~10-20 degrees below normal)
sets the stage for a potentially heavy upslope snow event over the
North-Central Rockies/High Plains through midweek. Some
deformation and terrain enhanced precipitation is also possible
over the Great Basin. Confidence is increasing on a moderate rain
focus shifting into the Plains Thursday-Friday.
Meanwhile, a wavy frontal boundary will settle/linger this week
into the northern Gulf of Mexico and focus periods of moderate to
heavy rains over the Florida/Gulf Coast/Lower Mississippi
Valley/Southern Plains through the entire period. Amounts don't
look very high, but the Central Gulf Coast states have had much
above normal rainfall recently, so there may be flooding concerns.
Farther north, guidance has noticeably trended upward with respect
to rainfall across parts of the East Coast/Northeast
Thursday-Friday. Some models suggest moderate to locally heavy
rainfall to accompany the cold front/surface low as it traverses
the region. Coastal low genesis may also present some
maritime/coastal threats across New England late week/next
weekend.
Santorelli
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml