Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1113 AM EDT Sun Jun 16 2019
Valid 12Z Wed Jun 19 2019 - 12Z Sun Jun 23 2019
15 UTC Update...
A blend of the ECMWF/GFS was used as a basis for this forecast
update during days 3-4 (Wed-Thu). A couple waves of low pressure
are forecast to traverse a quasi-stationary surface front from the
Central Plains to the Mid-Atlantic during this time frame. Given
the relatively small scale of the upper shortwaves associated with
these features, models showed timing differences, and a
consensus/middle ground approach was followed, best reflected by a
blend of the 00Z ECMWF/GFS solutions. Additionally, an amplifying
upper-level trough across the Northwest/northern Rockies during
this time frame was handled relatively similarly by the
aforementioned solutions.
By days 5-7 (Fri-Sun), model differences begin to increase and
confidence in the specifics of the forecast quickly decreases.
Timing/amplitude differences develop with the leading shortwave in
the western trough, and a trailing shortwave diving southeast
along the western Canada/northwestern U.S. coast also showed
significant model as well as run-to-run timing and amplitude
differences. Downstream implications of these differences are
significant with respect to the low pressure system expected to be
crossing the Plains/Midwest by next weekend, and solutions for
this system varied widely. Ensemble members did show some
semblance of clustering, however, lending more credibility to the
means. Farther east, models continued to indicate the potential
for some phasing of northern/southern stream features near the
Northeast U.S. coastline by Fri-Sat, with the potential for
development of a more pronounced frontal wave - although solutions
differed substantially on the timing of this process. Given these
considerations, opted to trend substantially toward ECENS/NAEFS
ensemble means by days 5-7, with the forecast comprised entirely
of ensemble means by day 7.
Ryan
Previous Discussion (issued at 0440 UTC)...
...Overview and Guidance/Predictability Assessment...
Latest guidance indicates a persistent southeast Canada trough
with confluent flow south in the the MS Valley and Oh Valley to
the northeast and mid Atlantic. This allows multiple frontal waves
to progress across these areas. The models have similar
depictions with different timing of the waves. For days 3-4 the
18z GFS and 12z UKMET are on the slow side of the guidance spread
with the wave progressing from the mid MS valley Wed to the Oh
Valley Thu and New England Fri. Given the faster jet usually
promotes faster wave progression, more weighting was given the 12z
ECMWF/ECMWF Ensemble Mean/12z Canadian/18z GEFS Mean solutions.
Consensus shows a closed anticyclone over the Pacific west of
northern CA that remains slowly moving through next weekend.
Teleconnections from this anomalously strong high in the northeast
Pacific lead to a downstream trough over the northern Plains with
an embedded closed low near the US Canadian border through the
middle of next week before retreating into southern Canada next
weekend.
Prefer a blended approach giving more weight to the ensemble means
of the 12z ECMWF and 18z GEFS Ensemble Means, with less weighting
to the operational 12z/18z GFS, which forecast a more progressive
upper trough than the 12z ECMWF/Canadian and the ensemble means of
the 18z GEFS and 12z ECMWF. The GFS is known to have a fast bias
in moving closed lows aloft, which is suspected to be the case
here, especially with the GEFS Mean slower and in much better
agreement with the ECMWF and ECMWF Ensemble Mean.
The 18z GEFS Mean and 12z ECMWF Ensemble Means continue to show
better than average agreement over the northern Rockies and
northern Plains, so more weighting was given to the Ensemble Means
of the 18z GEFS and 12z ECMWF.
...Sensible Weather Highlights/Threats...
The expected progression of waves of low pressure along a front
continues to be favorable for multiple episodes of convection from
the southern half of the Plains across the lower-mid MS Valley,
followed by the Ohio Valley/Tn Valley/ Appalachians/Mid
Atlantic/Northeast.
The Florida peninsula should see diurnal convection day 3-6,
possibly declining day 7 as an upper ridge strengthens.
During Wed-Thu expect showers/storms to spread into the northern
Plains/Upper Mississippi Valley with the system forecast to affect
those regions, with another round of showers/storms near where
moisture and instability focus across eastern OK/AR/TN/northern
MS/AL/GA.
From Fri 21 Jun-Sun 23 Jun the showers/storms may extend from
eastern OK across the mid-upper MS Valley into the Great Lakes and
Ohio Valley, with more than one round possible as the frontal
system moves slowly east in the blocking pattern.
Over the West, expect broad coverage of above normal temperatures
Wed and Thu in CA/OR/NV with some pockets of greater than plus 10F
anomalies centered on interior northern CA.
The anomalies might taper a bit but temps remain above normal from
CA into the southwest Sun 23 Jun. The other area of persistent
above normal temperatures is expected to be Texas.
WA and ID/northern OR cool with height falls in the middle of the
week, remaining cool through next weekend.
Areas across the northern Plains starting with eastern MT and WY
and spreading into the western Dakotas will see below normal highs
by as much as 10 to 15 degrees below normal Wed 19 Jun through Sun
23 Jun. The central US trough builds up the southeast ridge with
above normal temps next weekend expected for coastal SC across
southeast GA and much of Fl.
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