Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1130 AM EDT Sun May 12 2019
Valid 12Z Wed May 15 2019 - 12Z Sun May 19 2019
1530 UTC Update...
A blend of the 00Z ECMWF/CMC and 06Z GFS was used as a forecast
starting point through days 3-5 (Wed-Fri) for this update. The
largest discrepancies among the guidance during the medium range
pertained to the progression of a deep western U.S. trough
eastward toward the central U.S. days 6-7 (Sat-Sun), and the
degree of upper ridging that builds across the Southeast during
the same time period. Models differ on timing as well as intensity
of the upper trough by next weekend as it moves from the Four
Corners region toward the Plains, with the CMC on the slower side
of the progression and the ECMWF/GFS on the faster side.
Spread/trends among ensemble members suggest that either of these
scenarios remain possible favoring a continued blend of the three
aforementioned deterministic solutions along with increased weight
on ensemble means by that time period. Nonetheless, development of
a surface low (or series of waves of low pressure) along a fairly
strong frontal zone across the north central U.S. appears likely
by late in the week and continuing through the weekend, with a
slow-moving cold front/synoptically reinforced dry line persisting
across the Central/Southern Plains. Farther east, the ECMWF/CMC
have trended toward building a much stronger upper ridge across
the Southeast next weekend over the past couple runs, while the
GFS (as of 06Z) has not joined. A look at teleconnections
associated with some of the more substantial hemispheric 500 hPa
height anomalies shows little in the way of support for a ridge as
strong as the ECMWF/CMC show. On the other hand, did not want to
completely ignore the trend. Thus, continued with use of small
amounts of ECMWF/CMC/GFS through day 7 with majority weight placed
on the NAEFS/ECENS ensemble means.
Ryan
Previous Discussion (issued at 0509 UTC)...
Tonight's forecasts were derived from a blend of the latest
deterministic 12z ECMWF blended with the 12z ECMWF/18z GEFS
Ensemble Means days 3-7, with less weighting on the 12z
deterministic Canadian global model.
Models are slowly converging on the latitude of the low developing
off the New England coast Mon night-Tue. The 12z ECMWF/ECMWF
Ensemble Mean/12z UKMET/12z Canadian global remain farther north
than the GFS/18z GEFS Mean Wed. The forecasts gave more weighting
to the majority cluster, and less to the operational 18z gfs, now
the southern most solution. This system moves across the Canadian
maritimes and adjacent coastal waters Thu.
Elsewhere, models have typical timing/amplitude differences with
northern stream energy crossing the Midwest/Great Lakes Wed and
crossing the central Appalachians/mid Atlantic/coastal waters Thu,
with potential low pressure development off the coast Fri 17 May.
There remains a large latitudinal spread as to where low pressure
develops on the surface front. A blend of the 12z Canadian/ECMWF
models and the 18z GEFS/12z ECMWF Ensemble means was used until
better agreement develops.
Agreement amongst the models/ensembles supports an amplified upper
trough reaching the West Coast by Wed 15 May, with a secondary
trough reaching the west coast Thu 16 May, which then moves inland
into the Great Basin and towards the Pacific northwest coast Fri
17 May. Models differ on the exact timing and phasing of the
trough, with the ECMWF/ECMWF ensemble mean faster taking the upper
trough across the Rockies on to the high Plains. Blending with the
slower timing of the GEFS was used following historically slower
motion of higher amplitude upper troughs. The 18z GFS was
excluded from the blend as the upstream east Pacific Ocean low was
half a day behind the pack of the other models/means Thu 16 May to
12z Fri 17 May.
Height falls crossing the Rockies on to the Plains by late next
week should induce surface cyclogenesis across the northern Plains
Fri night-Sat, drifting slow east northeast towards the upper MS
Valley Sun 19 May. Good clustering among the 12z ECMWF/18z GEFS
Mean/12z ECMWF Ensemble Mean led to a consensus of these
forecasts. The 00z GFS clusters well with this group of forecasts
low positions as well.
...Weather Highlights/Threats...
Expect showers over the New England mid week as low pressure is
slow to move off the northeast coast. Some of the precipitation
could fall in the form of snow over highest elevations of northern
New Hampshire and western Maine as the core of the upper trough
passes through.
A cold front and frontal wave moving east across the Upper MS
Valley, Upper Midwest/Great Lakes/Ohio Valley should produce
showers during the early to middle portions of next week before
the system moves into the northeast later in the week.
Showers/storms also develop in the vicinity of a stationary front
lingering over the southern Fl peninsula in the middle to latter
portions of next week.
The upper trough nearing the West Coast should bring increased
clouds/precipitation to the Pacific Northwest and northern CA Wed
15 May, continuing inland into the Great Basin and northern
Rockies Thu 16 May, and then the Plains on Fri 17 May and Sat 18
May. Higher elevation snows develop as colder air aloft moves
into the northwest US, starting in northern CA/western OR Thu and
then spreading into the higher elevations of the Northern Rockies
and northern Great Basin next weekend.
With low pressure moving slowly east across the Plains next
weekend, return moist southerly flow provides fuel to enhance
shower/thunderstorm activity, with potential for locally heavy
rain to develop over portions of central Texas.
Temperature-wise, highs of 10-15 F above normal are forecast over
MT/WY/western Dakotas Wed. The warm core moves east into the
northern to central Plains Thu with cooling over the northwest.
With the approaching trough, CA/OR/WA will see a reversal to
cooler than normal temperatures in the middle to latter portions
of next week.
The system affecting the northeast Wed will keep highs 10-15F or
so below normal over northern New England. Gradual moderation is
expected as the trough moves offshore Thu 16 May, with slightly
above temperature Fri into the weekend. Temperatures several
degrees above normal develop over the southeast late in the week
and continue through the weekend.
Petersen
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