Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Tue Apr 02 2019
Valid 12Z Fri Apr 05 2019 - 12Z Tue Apr 09 2019
...Overview...
Expect continued moderate progression of Pacific flow into/across
the Lower 48 during the period. This pattern should bring
multiple episodes of precipitation to areas from the
northern-central West Coast into the northern Rockies, while the
eastern half of the country will likely see two broad areas of
moisture--one with a fairly weak leading system crossing the East
and another with a system tracking from the southern Plains
northeastward during the weekend and early next week. To the east
of the Rockies, best potential for highest rainfall totals should
be within an area extending from Texas through the Lower
Mississippi Valley into the Southeast/southern Appalachians. Some
areas farther north could see significant rainfall as well.
...Guidance Assessment/Preferences...
During the first couple days or so of the period a
multi-model/multi-run consensus (past two 12-hourly ECMWF runs and
6-hourly GFS runs through the 12-18Z cycles, with some 12Z CMC
included) represented the most likely solution across the Lower 48
and vicinity. Guidance agrees in principle on individual features
but shows typical differences for smaller details that have low
predictability. The system initially over the East as of Fri
still looks diffuse aloft and as a result models are having
trouble resolving surface specifics. Long-term trend has been
toward a weaker surface reflection which was advertised earlier by
ECMWF-based guidance. There is still a lot of spread though.
Meanwhile a strong storm to the west/northwest of the Pacific
Northwest will bring a front into the West Fri-Sat and into the
Plains by Sun. Trailing bursts of energy aloft/surface features
should continue into the Northwest through the weekend. The 12Z
UKMET seemed to differ the greatest from other solutions for
details over the eastern Pacific/Northwest U.S. and thus was
excluded from the blend. Leading shortwave energy over the West
late this week will focus in the southern stream as it heads east,
reaching the southern Plains by Sat.
By the latter half of the period there are two prominent overall
uncertainties: 1. How energy reaching the West on Sat ultimately
evolves and interacts with leading southern stream shortwave
energy over the Plains--affecting the forecast over the eastern
half of the country, 2. The strength of ridging that builds into
the West by Sun-Tue and resulting speed of an upstream eastern
Pacific trough. For this latter western issue, the majority of
guidance shows less pronounced western ridging and faster Pacific
trough versus the GFS/GEFS runs from the past couple day. A
positive height anomaly center to the east of Greenland and a
negative height anomaly center over the west-central Pacific in
recent D+8 charts teleconnect to a split-flow pattern over western
North America (mean ridging generally confined to Canada and
northern parts of the western U.S.). This seems to favor an
evolution closer to the ECMWF/CMC and their means, at least in
terms of having lower heights than the GFS/GEFS over southern
parts of the West. Farther east GFS runs vary from
consensus/means in varying ways, the 18Z GFS straying on the fast
side and the new 00Z run becoming much stronger with northern tier
energy aloft. However confidence in other model details is not
great either. These considerations favor trending the forecast
toward a blend of recent ECMWF runs, 12Z CMC, and 12Z NAEFS/ECMWF
means. The NAEFS provides a modest hedge to account for the
possibility of slightly slower Pacific/western U.S. progression.
...Weather Highlights/Hazards...
Continue to expect periods of enhanced rainfall/higher elevation
snow from the central/northern West Coast into the northern
Rockies. Some locations along the most favored terrain from
northern California/Sierra Nevada into western Washington/Oregon
may see at least several inches liquid over the Fri-Tue period.
The northern Rockies should see lesser but still significant
totals. Individual systems affecting the Pacific Northwest coast
may also be accompanied by periods of strong winds. Confidence in
precipitation coverage and intensity decreases during the first
half of next week as guidance diverges for amplitude/timing of
significant features.
Leading area of precipitation over the East, possibly including
some snow over New England, should depart after Fri night. Some
pockets of locally heavier precipitation are possible but there is
not a clear signal for where such activity may occur. From Fri
night onward the shortwave aloft approaching/reaching the southern
Plains along with strengthening flow of Gulf moisture interacting
with a stalled front will likely lead to a heavy rainfall threat
over the southern Plains/Lower Mississippi Valley and possibly
extending to the east/northeast. Low pressure tracking
northeastward from the southern Plains should eventually help to
spread moisture over much of the eastern half of the country.
Depending on how this system evolves there may be some bands of
significant rainfall in middle/northern latitudes between the
Plains and East Coast. Plains/Mississippi Valley areas sensitive
to additional rainfall should monitor forecasts for this system.
The overall pattern should favor modestly below normal highs over
the West Coast states into the northern Rockies but with a warming
trend over southern areas. Lows should be above normal through
the period (some plus 10-15F anomalies). Most areas east of the
Rockies should see above to much above normal temperatures, though
with a cooling trend behind the front reaching the eastern states
late in the period. Morning lows may be 15-25F above normal over
parts of the northern Plains and Midwest around Sun-Mon with some
daily record warm low values possible.
Rausch
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