Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Tue May 22 2018
Valid 12Z Fri May 25 2018 - 12Z Tue May 29 2018
16Z Update:
The guidance is hinting at an omega block pattern trying to
develop over the continental U.S. by early next week, with a very
slow moving upper low over the Intermountain West and a broad
upper trough over the southeast U.S., separated by an upper ridge
over the Plains. In terms of the latest guidance, the 00Z UKMET
appears too quick with height falls across southern Canada on
Friday and Saturday, with the other guidance suggesting more in
the way of ridging. The CMC continues to be stronger with the
tropical disturbance over the Gulf of Mexico, although close to
the model consensus in terms of placement over the weekend. Both
the operational GFS and ECMWF have trended stronger with the
western U.S. closed low over the past 36 hours, with the ECMWF
indicating the most consistent signal. By the end of the forecast
period, greater weighting was applied to the EC mean while still
keeping some details from the operational guidance. The GEFS mean
has lower heights over the eastern Pacific compared to the model
consensus by the weekend and more ensemble member spread than the
EC mean, and thus its percentage in the forecast is less. The
previous extended forecast discussion is appended below for
reference.
Hamrick
...Guidance and Predictability Assessment...
Guidance variability and spread have tempered confidence for
various aspects of the forecast. Faster trends have been evident
for a while with the upper low moving into the West, with 12z/18z
runs continuing that tendency relative to prior days and new 00z
runs thus far holding onto a similar timing into day 5 Sun. Upper
low track has gravitated toward the southeast portion of the
spread but not yet as far south as recent UKMET runs. Later in
the period recent guidance diverged with this feature, the 12z
ECMWF actually pulling it southwestward while the 12z CMC showed
continued northeast progression. GFS runs were between the two
ideas. Some of this late period spread is connected to the
uncertainty in the character of upstream flow. Sporadic GFS runs
have been quite eager to develop an amplified trough to the south
of an upper low that settles into or near the Gulf of Alaska while
other solutions tend to be flatter with cyclonic flow or farther
west with troughing. Consecutive ECMWF runs have been very
different though and result in two opposing solutions for the
western low. Latest signals, reinforced by the 00z GFS/CMC, are
toward the idea that by early next week the western upper
low/trough may become slow to move in light of westerlies being
sufficiently far removed from the feature.
Farther east the past day of guidance has featured some
significant adjustments. Most solutions become noticeably weaker
with the shortwave initially over the northern Plains/southern
Canada as well as with energy flowing into western Canada. The
end result of these changes is a dampening of the associated
surface low with time. Meanwhile consensus (not including the
UKMET) showing higher heights over central Canada leads to a
potential trough aloft crossing eastern Canada/northeast U.S.,
with its passage leaving high pressure over New England where low
pressure/frontal system had been expected by Mon in previous
guidance. By mid-late period the volatility of guidance at least
favors leaning away from recent GFS runs that were strongest with
the upper trough/surface high pressure, but consensus is
sufficiently pronounced to incorporate the new scenario in the
updated forecast.
The forecast provides plenty of question marks from the northwest
Caribbean into the southern/southeastern U.S. as well. There is
decent agreement that some degree of low pressure should form but
spread persists regarding whether the system will gravitate toward
the core of an upper trough that deepens during the first half of
the period before stabilizing or weakening a little, or track
around the eastern side of the upper trough. Thus far it appears
the higher probability is closer to the central/east-central Gulf
track based on guidance comparisons to a feature initially
analyzed over the northwestern Caribbean at 00z. This leads to
leaning away from GFS/GEFS solutions. The National Hurricane
Center is issuing special tropical weather outlooks for this
potential system.
Based on guidance comparisons through 12z/18z, the updated
forecast incorporated the most common/favored ideas among the 18z
GFS, past two ECMWF runs, 00z CMC and 12z NAEFS/ECMWF means
early-mid period--followed by greater emphasis on the two means
while phasing out GFS/CMC input but keeping a minority weight of
ECMWF data.
...Weather/Threats Highlights...
Through a combination of a potential system emerging from the
northwest Caribbean/Gulf and the broad flow of moisture to the
east of an upper trough/low becoming established along 90W
longitude, heavy rainfall potential remains a concern for parts of
the southern/eastern U.S. This is especially the case over areas
that received significant rainfall totals over the past 1-2 weeks.
Farther north expect some activity with a gradually weakening low
pressure/frontal system with localized pockets of heavier
rainfall. Meanwhile the upper low moving into the West will be
responsible for another area of active weather during the period,
from the central/north-central west coast into northern-central
High Plains. Potential for slow movement of the upper low toward
early next week could lead to enhanced rainfall over some
locations.
Expect the most persistent warmth relative to normal to be over
the northern Plains/upper MS Valley with multiple days of plus 10F
or greater anomalies for min/max temps. Elsewhere east of the
Rockies min temps will generally be above normal but clouds/rain
over the southeast will keep that part of the country somewhat
below normal for highs. Onshore flow may bring a cooling trend
for highs over portions of New England and the Mid Atlantic by
early next week. The upper system moving into the west should
bring a period of below normal highs to California and Great Basin
especially from Fri through the weekend.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook 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
Rausch