Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Mon May 07 2018
Valid 12Z Thu May 10 2018 - 12Z Mon May 14 2018
...Pattern overview...
Wavy, low amplitude flow will initially set up over the U.S. with
a pair of shortwaves situated over the Great Lakes and Pacific
Northwest, respectively. While the former should carry a cold
front off the Eastern Seaboard by 11/1200Z, the latter height
falls will likely continue digging into the southwestern states
into Saturday. The mid/upper-level flow over the eastern Pacific
and western U.S. is forecast to become quite amplified from Day
5/May 12 onward. Each such 500-mb anomaly center will likely
depart around 1.5 to 2 standard deviations from climatology. This
strong upper low positioned across the vicinity of Utah on
Saturday morning should gradually trek eastward in time possibly
reaching the Mississippi River valley by next Monday/May 14. In
its wake, some remnant ridging is expected to slide over the
Pacific Northwest with weak troughing potentially sitting back
over the Four Corners region.
...Guidance/uncertainty assessment/preferences...
While the available guidance exhibits decent agreement with the
initial shortwave/attendant frontal zone crossing the East Coast,
more variability exists with amplifying flow over the southwestern
U.S. Multi-day operational/ensemble trends have shifted toward a
stronger and more closed off solution. As a result, there has also
been a push toward a more pronounced upper ridge across the
eastern Pacific. Through Day 5/May 12, with the exception of the
00Z CMC, the models are in reasonably good agreement. Uncertainty
begin to build thereafter as a wavy frontal zone and attendant dry
line are depicted differently depending on the model. There is a
notable difference in continuity between the 06Z/00Z GFS runs with
the latter being quicker to eject an embedded shortwave into the
Upper Midwest by Sunday morning. From this point onward, the
forecast becomes more chaotic per ensemble spaghetti plot
analysis. While multi-cycle trends of such plots have shown a
decrease in spread, uncertainty still plagues the Day 6/7, May
13/14 period. The 00Z/12Z ECMWF runs have favored a more
progressive scenario while generally maintaining the western ridge
into Day 7/May 14. Meanwhile, the 06Z/00Z GFS solutions favor a
bit more shearing of this trough with residual troughing possibly
lingering back toward the Desert Southwest. These general
scenarios are within the realm of possibilities but do reside on
the more extreme side of the ensemble plots. This would support a
push toward ensemble mean usage late in the period.
Through early Saturday, was able to generally use a
multi-operational model blend led by the 06Z GFS and 00Z
ECMWF/UKMET solutions. With spread growing markedly thereafter,
quickly pushed toward an ensemble mean based approach with more
emphasis on the 06Z GEFS mean relative to the 00Z ECMWF ensemble
mean given the preference for modest ridging over the western
states by Day 7/May 14.
...Sensible weather/threats highlights...
Throughout much of the forecast, below average temperatures should
situate over the Upper Intermountain West eastward into the Upper
Great Lakes region. During the first few days of the period,
multiple chances for overrunning precipitation will likely keep
numbers on the cooler side. This would generally favor readings in
the 50s as departures from climatology sit around the 5 to 10
degree mark. Even some departures approaching the 10 to 15 degree
range are possible on Saturday across the central High Plains as
the upper low nears the region. Except for cool weather likely
across interior New England, much of the rest of the country
should anticipate above average readings into early next week.
Numbers approaching the century mark are possible over the
southern High Plains through Saturday, generally juxtaposed with
the meandering dry line. Such temperatures would suggest 20 degree
departures from average.
The initial threat for moderate to locally heavy precipitation
will reside across the north-central U.S. while eventually
shifting focus toward Wyoming as the upper low approaches. Cold
temperatures aloft would support some snowfall accumulations
across the higher elevations of the northern/central Rockies. As
this system advances toward the Great Plains, expect another swath
of convection to break out over the middle of the country this
weekend. This threat would gradually migrate eastward depending on
the timing of the frontal zone. Elsewhere, a sufficient surge of
high precipitable water air will increase the heavy rainfall
threat over Florida. The 06Z GFS showed such values reaching the 2
inch mark, possibly higher.
Rubin-Oster
Associated WPC medium range forecasts of 500 mb heights, surface
systems, weather grids, quantitative precipitation forecasts, and
winter weather outlook snow/sleet 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