Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1208 PM EDT Sat Apr 27 2019
Valid 12Z Tue Apr 30 2019 - 12Z Sat May 04 2019
...Heavy rain and the potential for flooding and flash flooding
are expected for parts of the Southern Plains and Lower/Middle
Mississippi Valley...
...Overview and Assessment of Model/Ensemble Guidance and
Predictability...
A large upper low located in central Canada on day 3/Tue will
retrograde westward through the medium range period, encroaching
upon an upper ridge set up in Alaska and into the Arctic.
Uncertainty remains regarding the orientation of the low and vort
maxes within it, as well as the amount of energy that could stream
underneath the aforementioned ridge. The 00Z deterministic ECMWF
is the most bullish with that energy by day 6/Fri, which
suppressed ridging in the northern Pacific. This was unsupported
by other models and ensembles, so leaned away from that solution
for now.
Across the CONUS, a general pattern of upper troughing will remain
across the West through midweek, while a subtropical ridge
persists over the Southeast. A shortwave is expected to strengthen
and move through the Midwest by Thursday. Additional energy may
push through the southern stream across the Southern Plains/Lower
Mississippi Valley at that point too, with the 00Z and 06Z GFS
runs more bullish with this feature than the EC. Nevertheless,
expect the upper trough to push eastward late in the week with
these features, leading to high pressure at the surface for the
central and eastern parts of the country by Fri/Sat. Model
disagreement increases by day 6 and especially day 7/Sat for an
upper low in the eastern Pacific approaching the West Coast; the
00Z EC keeps the low in the southern stream, while particularly
the 00Z GFS combines the northern and southern streams and leads
to a more progressive trough in the West. At this point, a
compromise seems reasonable, but leaned toward the EC mean a bit
as the GFS has more run-to-run variability and is often too fast.
Overall, a multi-model mainly deterministic blend seemed like the
best solution early in the period, gradually transitioning to
favoring the EC and GEFS means by late in the week.
...Weather Highlights/Threats...
With the troughing in the West early in the period, temperatures
will be cold enough to support snow mainly in the Northern/Central
Rockies, though late season light snows are possible in the
Northern Plains. The main threat in the medium range, however, is
for heavy rainfall developing in the center of the country as
moisture streams in ahead of the trough and combines with a
slow-moving/wavy front. Multi-day totals are expected to be
several inches for the Southern Plains and the Lower/Middle
Mississippi Valley as multiple rounds of rain and thunderstorms
(potentially severe) will occur. Flooding remains possible as
particularly the Mississippi Valley has had flooding concerns for
weeks, and with the convective nature of the precipitation, flash
flooding is possible as well. There is some potential for heavy
rain to spread over the Ohio Valley and Great Lakes region at
times as well. The front is expected to push eastward ahead of the
trough by Fri/Sat, helping lessen the heavy rain threat.
Tate
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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