Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 04/16/20

Area Forecast Discussion

National Weather Service Des Moines IA

 927 PM CDT Wed Apr 15 2020

...Update Regarding Winter Weather Headlines...

.UPDATE...
Issued at 913 PM CDT Wed Apr 15 2020

Strong frontogenetical forcing continues to be favored across the
Hwy. 34 corridor Thursday across southern Iowa. Lift through the
DGZ will be quite strong, and thus will continue to support an
environment capable of heavy snowfall. As a result, the Winter
Storm Watch has been upgraded to a Winter Storm Warning for the
southern two tiers of counties in Iowa, with a Winter Weather
Advisory in the row of counties north of it. The biggest challenge
was deciding whether or not an extra northward row of counties
needed to be added to the Winter Weather Advisory. 00z CAM
guidance this evening has continued to place a sharp cutoff in
snowfall totals, due to a dry slot that sets up between
Interstate 80 and Hwy. 30. This guidance does still depict a
weak area frontogenesis to move across the northern extent of the
snowfall. The one key change in guidance this evening has been a
delay in the snowfall across at the northern end of the gradient,
with most solutions not bringing heaviest precipitation until
after 18z. Thus, given this delay further north, have elected to
hold off on taking the Winter Weather Advisory further north, and
will reanalyze when the complete set of 00z guidance is available.
If a shift northward continues to trend, the overnight shift can
then assess the need to expand the Winter Weather Advisory. For
the counties in the Winter Storm Warning in southern Iowa, CAMs
depict an onset time for precipitation after 10z, as the
frontogenesis begins to ramp up along the Hwy. 34 corridor.

&&

.DISCUSSION.../Tonight through Wednesday/
Issued at 346 PM CDT Wed Apr 15 2020

The snow overnight Tuesday into this morning aside, we couldn`t
escape a day without some flurries (or sprinkles) across parts of
central Iowa. HRRR and RAP soundings show the moisture depth a bit
greater than yesterday`s model runs and within the dendritic growth
zone (DGZ) with some elevated instability. While not as robust as
Monday or Tuesday with large scale subsidence overhead, this has
allowed for flurries or sprinkles to pass over portions of our
northwest forecast area into our central and southeast counties this
afternoon. Unfortunately, this will not be the last snow mention in
this discussion.

While a broad trough remains carved out across the CONUS, an
embedded, potent shortwave coming into the Pacific Northwest this
afternoon will be the feature that will bring a mid-April snow
episode to southern Iowa tomorrow into tomorrow night. As this
shortwave digs into the central Rockies, surface low pressure will
develop over the southern Plains with a baroclinic zone setting up
over eastern Kansas into Missouri. This will promote an excellent
overrunning precipitation event with isentropic lift primarily
focused in the 290 to 295K layer into northern Missouri and southern
and central Iowa. Once again, have lowered temperatures back into
the middle 30s across southern Iowa from initial guidance, which had
highs in the low 40s. Soundings continue to point toward the
temperature trending to the wet bulb once precipitation begins.
Further support for lowering temperatures will be the snow rates.
The 3z/9z SREF and 0z/12z HREF 1"/hr snow rates have a high
probability over southern Iowa during the day Thursday. These rates
are supported by frontogenetical forcing that is maximized in the
low levels with cross sections showing moderate to strong omega
within the DGZ. These rates may be enhanced by weak convective
instability in the south per HRRR sounding at LWD. With these snow
rates, should be able to dynamically cool the column for all snow
over southern Iowa with the rates also supporting accumulation on
roadways despite the date being in mid-April. There could be some
changeover or rain mix on the northern side of the precipitation
shield over central Iowa along and north of I-80 where lighter
precipitation may not foster as much dynamic cooling.

Looking at a variety of QPF guidance from EC/EPS, GFS/GEFS, HREF,
and individual models, the mesoscale band of precipitation should
set up over southern Iowa or far northern Missouri. However, the
forecast is likely still a bit too broad with the QPF and not high
enough in a narrow, yet to be determined corridor to reflect the
higher end snow amounts. HREF probability matched mean and ensemble
max show a several county area over southern Iowa with 10 to 12" and
12 to 15", respectively. Those higher amounts may be a bit too broad
in spatial extent, but do indicate the upper end potential in this
mesoscale banding event. On the northern edge of the precipitation,
there will also be a tighter gradient with rain mixing in at times
and also fighting some lower level dry air. This will put a gradient
across the Des Moines metro and other I-80 counties with several
inches possible in the southern part and an inch or less in the
northern part of these counties.

Headline wise is a quandary. This is a solid advisory for southern
Iowa, but with the upper end potential and current forecast near 10"
along the Iowa Missouri border, certainly warrants a watch. While
this will leave little lead time when the watch is changed to an
advisory and/or warning with precipitation commencing in our
southwest area, it will allow for a further tightening of the
gradient and fine tuning of the location of the expected highest
snow totals.

As the snow ends early Friday, the pattern will take on a more
spring-like pattern with rising heights as high pressure passes
south of the region Friday into Saturday. While another cold front
will pass through on Sunday, it is primarily moisture-starved with
only perhaps a slight impact on temperatures on Sunday across the
north. Temperatures will be fairly close to typical for mid-April
for the weekend into early next week. The next chance for showers or
even some thunderstorms will arrive on Tuesday with the highest
chances over southern Iowa.

&&

.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 627 PM CDT Wed Apr 15 2020

Quiet weather for the rest of this evening. Cumulus clouds with
VFR bases will dissipate. Low pressure will track across central
Missouri tomorrow, and will bring heavy snowfall to southern Iowa.
The main terminal impacts will to OTM, which will see visibility
less than 2 miles at times if the one inch per hour snowfall rates
are fully realized. DSM is the most uncertain TAF, as a sharp
cutoff in dry air will be present near the Des Moines metro. For
now, will leave the mention of snow and slightly reduced
visibility in the TAF. If the banding snow shifts northward, the
impacts could be greater, but a shift few miles further south
could potentially leave DSM mostly dry. The precipitation will
stay well south of FOD, ALO, and MCW.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 5 AM Thursday to 7 AM CDT Friday for
IAZ081>086-092>097.

Winter Weather Advisory from 5 AM Thursday to 7 AM CDT Friday
for IAZ070>075.

&&

$$

UPDATE...Krull
DISCUSSION...Ansorge
AVIATION...Krull



AREA FORECAST DISCUSSION

National Weather Service Kansas City/Pleasant Hill MO

 914 PM CDT Wed Apr 15 2020

.Update...
Issued at 913 PM CDT WED APR 15 2020

Have upgraded the winter storm watch to a warning and added a an
advisory for a tier of counties further south. Water vapor
imagery, along with radar imagery, show a fairly strong and
separate mid-level shortwave trough moving across Nebraska.
Forecast sounding across northern MO show a sizable layer of dry
air in the low levels. But there is stronger vertical motions in
this wave, noted by the lighting being detected, and the drier air
will allow temperatures to cool relatively quickly to the wetbulb
temperature. Given this, think that between 09Z and 12Z we`ll see
a band of snow develop across northern MO. This band should be
relatively light, perhaps a dusting to an inch. There is also some
uncertainty as to if this band will dissipate or persist before
the the main ascent kicks in from the strong wave later in the
day. It`s this main wave, and the very strong ascent, that will
lead to the development of the main band of heavy snow. Strong
frontogenesis with strong isentropic ascent coupled with favorable
upper level jet dynamics tomorrow evening could lead to very
heavy snow across northern MO along the IA state line.

&&

.Discussion...
Issued at 424 PM CDT WED APR 15 2020

Major forecast concern today revolves around a quickly evolving and
potentially quickly escalating system as it ejects into the central
part of the CONUS by Thursday. At the surface a strong ridge is in
place across Minnesota and Iowa, with its southern periphery nosing
southward toward the I-44 corridor. A notable boundary is evident in
both visible satellite imagery and surface observations in
southeastern Kansas and southwestern Missouri. Temperatures on the
cool side of this boundary have struggled to get above 50 despite
ample sunshine, so daytime highs have come up a little short of
forecast temps. This surface ridge will eventually give way to a
fairly deep surface and low level trough by tomorrow and that should
start the process of advecting warm/moist air into the immediate
forecast area.

The forecast for Thursday will be pretty tricky from a temperature
standpoint as well as the obvious concern for moderate to heavy
snow. A surface warm front will likely set up somewhere near
(probably south of) the I-70 corridor, leaving the northern half of
the CWA in the much cooler regime. Temperatures along and south of I-
70 may reach 60 degrees, but areas north of that warm front will
struggle to get out of the 40s. Further north where precipitation
will be more prevalent it`s likely that surface temperatures will
hover around freezing through the day on Thursday into Friday. With
little support aloft, the main mechanism for ascent for this system
will be isentropic ascent. This is very evident as deep isentropic
ascent along the 290K to 300K surface. The resultant low level
frontogenesis will provide great and efficient lift for moderate to
at times heavy precipitation along and north of the better low level
convergence. Then comes the question of exactly where these low
level features will set up. Perhaps the most important feature to
watch will be the H85 0C isotherm, as that should give a good proxy
of where to focus on the heavy snow concerns. NAM forecasts have
indicated a slightly stronger H85 cyclone which is pushing the H85
warm front a little further north, into southern Iowa. This would
result in perhaps some lower amounts in the Watch area. HRRR H85
cyclone looks to be a bit weaker and maybe slightly further south,
which may keep the watch area in slight cooler air, and maybe bigger
amounts. Given the high uncertainty, even at this juncture, have
opted for a Winter Storm Watch, but a decision to go with a warning
or advisory will need to be made fairly soon. Further to the south,
away from the better frontogenesis an advisory may be needed, as the
post H85 cyclone cool air comes in and gradually changes
precipitation over to snow, perhaps as far south as the I-70
corridor. This would likely take place Thursday afternoon and
evening, lasting into the early hours of Friday night, with a
dusting to perhaps 2 inches of snow.

Thereafter the pattern looks to be a bit on the quieter side with
temperatures coming back up near or maybe even above seasonal
normals.

&&

.Aviation...(For the 00Z TAFS through 00Z Thursday Evening)
Issued at 626 PM CDT WED APR 15 2020

While IFR conditions will prevail through the overnight hours,
conditions are expected to deteriorate by late tomorrow morning or
early tomorrow afternoon. Low ceilings with rainfall will move
into east central KS and western MO, with snow increasing across
northern MO. Ceilings are expected to become at least low MVFR and
could possibly become IFR by the afternoon.


&&

.EAX WATCHES/WARNINGS/ADVISORIES...
KS...Winter Weather Advisory from 5 AM Thursday to 7 AM CDT Friday
     for KSZ102.

MO...Winter Storm Warning from 5 AM Thursday to 7 AM CDT Friday for
     MOZ001>008-016-017.

     Winter Weather Advisory from 5 AM Thursday to 7 AM CDT Friday
     for MOZ011>015-023>025.

&&

$$

Update...CDB
Discussion...Leighton
Aviation...CDB




AREA FORECAST DISCUSSION

National Weather Service Jackson KY

 755 PM EDT Wed Apr 15 2020

.UPDATE...
Issued at 755 PM EDT WED APR 15 2020

Pretty good dewpoint depressions continue across east Kentucky
with many areas still seeing depressions of 30 or greater. This
suggests none of the returns we are seeing from our radar in the
north are likely reaching the ground, or at worst just a few
sprinkles. Thus, have backed off on precipitation chances. Also,
given these dewpoint depressions, we may be hard pressed to see
much frost, except in the north or northwest where slightly higher
dewpoints have spread into the area. We would have to get much
colder to get much frost based on where our cross over
temperatures will reside. However, freezing temperatures would
appear more likely given the drier air allowing temperatures to
drop off quickly late tonight. Thus, no changes planned to the
headlines, but will reduce the frost mention a bit.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 425 PM EDT WED APR 15 2020

19z sfc analysis shows high pressure currently over eastern
Kentucky with plenty of sunshine and light winds in the east.
However, a moisture starved cold front is pressing southeast
toward the area late this afternoon with a band of clouds and
light rain along with a period of gusty winds. Winds in our
northwestern parts are also responding to the approach with gusts
to 25 mph from the southwest noted. Meanwhile, the sunshine has
helped temperatures to climb into the low and mid 50s most places
with dewpoints in the very dry teens and low 20s. Any pcpn from
the front will also have to overcome this dryness to make it to
the sfc this evening, as well.

The models remain in very good agreement aloft through the short
term portion of the forecast. They all depict the deep trough over
the Great Lakes and points north bottoming out this evening with
heights falling a bit more overhead. This happens as an impulse
passes through the JKL CWA tonight. Heights will then start to
rebound as the trough pulls away - flattening the pattern into one
that is zonal and rather fast with additional weak energy packets
passing through. Lurking just the the northwest at the end of the
period will be another northern stream trough heading this way for
Friday. Given the minimal model spread today have again chosen to
go with the NBM as the starting point for the grids with some
adjustments toward the HRRR and NAM12 for near term details into
Thursday morning.

Sensible weather will feature another cold night thanks to a
secondary cold front passing through. While any pcpn associated
with this will dry out as it moves into the JKL CWA a few
locations could pick up a couple of hundredths of an inch of rain
from a light shower or sprinkles this evening into the first part
of the night. The clouds associated with this feature will be
fairly sparse, though, clearing out quickly in its wake north and
as it dissipates in the south towards dawn. As a result, another
round of sub-freezing temperatures are anticipated for much of the
area along with widespread frost everywhere. Accordingly, have
adjusted the earlier freeze warning to dip further south into the
CWA while leaving a Frost Advisory over the rest. Plenty of
sunshine on Thursday will erase the frost and freezing
temperatures early Thursday with temperatures rebounding from the
upper 20s and lower 30s to the low and mid 50s north and near 60
south. Despite this rebound, some of the sheltered valleys could
see temperatures low enough for some patchy frost by early Friday
morning. This will likely be a more traditional ridge to valley
temperature split night, as well, ahead of another inbound
frontal boundary.

Did adjust the temperatures down a tad from the NBM tonight and
also more significantly in the valleys on Thursday night. As for
PoPs, ended up a little higher and quicker than the NBM for the
passing front tonight - mainly favoring the northeast for any
measurable QPF.

.LONG TERM...(Friday through Wednesday)
Issued at 307 PM EDT WED APR 15 2020

The extended period will start off wet, as a slow moving frontal
boundary moves through the region Friday, Friday night, and most
of Saturday. This first system will exit the area late Saturday
afternoon. We will see only a short break from the rain, however,
as another weather system moving in from the southwest will bring
us another shot of rain Saturday night through very early Monday
morning. We should see an extended period of dry weather to begin
the new work week, as a ridge of high pressure settles over the
region from early Monday through early Tuesday evening. The good
news is we should see much warmer temperatures, with daily highs
averaging in the 60s for most locations. Nightly lows should be in
the upper 30s to upper 40s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
ISSUED AT 755 PM EDT WED APR 15 2020

VFR conditions will continue through the TAF forecast period.
Northwest winds between 5 to 10 knots will become light and
variable by late tonight. Those light winds will continue through
tomorrow.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
Freeze Warning from midnight tonight to 9 AM EDT Thursday for
KYZ044-050>052-058>060-068-069-079-080-083-104-106>120.

Frost Advisory from midnight tonight to 9 AM EDT Thursday for
KYZ084>088.

&&

$$

UPDATE...KAS
SHORT TERM...GREIF
LONG TERM...AR
AVIATION...KAS



Area Forecast Discussion

National Weather Service Marquette MI

 717 PM EDT Wed Apr 15 2020

.SHORT TERM...(This evening through Thursday)
Issued at 340 PM EDT WED APR 15 2020

Deep troughing over much of eastern North America continued today,
with convective snow showers developing in the W to NW flow across
the region. Webcams across the area have shown some light snow
showers dropping flakes down to the surface and overcoming the dry
low levels. For the most part though, much of the snow has been
evaporating before reaching the surface.

This afternoon, models have been consistent with a convergent band
developing across southern and eastern Marquette County into Alger
County and southward into Delta County beginning around 20z. The RAP
and HRRR appear most aggressive and we are already starting to see
some radar signatures of more organized convection. I`m skeptical of
how much snow would reach the surface though, given the dewpoint
depressions close 20F at KSAW. Cooling and thus moistening of the
column will be possible if precip rates are high enough, so have
kept scattered POPs in for now and maxed snow totals in this zone
around 1.0 to 1.5". Quick visibility reductions would be likely over
a short distance within this or any other snow showers. Aside from
this, expecting gradually clearing skies as the atmosphere stabilizes
some. GOES 16 Ch2 highlights that this is already occuring just west
of the Keweenaw. A decent radiational cooling night should take
place tonight given the clearing skies, light winds and PWATs 0.25-
0.10. Went with 10 percentile lows this evening to reflect this
which puts lows in the interior west 10F into the mid teens and by
the lakeshores in the high teens to very low 20s.

Thursday, a mid-level shortwave will move through the region.
Somewhat steep looking lapse rates coupled with additional forcing
provided by this shortwave, could develop snow showers similarly to
today, with a more convective signature. Greatest chances look to be
in the east within the west-northwest wind snow belts. Soundings
indicate continued dry low levels though, even in areas where lift
is maximized. Wetbulb zero heights indicate that any precip should
be snow. Given all this, I`m again skeptical of anything more than
isolated to scattered light snow showers developing and precip
reaching the ground should be limited. Highs tomorrow should be
slightly warmer than today in the 30s.

.LONG TERM...(Thursday night through Wednesday)
Issued at 428 PM EDT WED APR 15 2020

Models suggest that ridging will persist from the Gulf of Alaska
into western Canada resulting in downstream troughing over eastern
North America. Mainly northwest flow will prevail through the
northern Great Lakes with periodic shortwaves brushing the area.
Temps will remain generally below average with a short period of
more moderate conditions into the weekend.

Thu night into Fri, Even with 850 mb temps remaining near -10C,
lower inversion heights with 700 mb temps around -17C will be less
favorable for either overnight LES or another round of diurnal shsn.

Sat-Mon, WAA with strengthening sw flow ahead of an approaching
shrtwv and sfc trough will help push temps to around 50 even as
clouds increase. The shrtwv and associated cold front will move
through Sat night bringing a chance of rain showers, possibly
changing to snow. Colder and drier air will move back in Sunday with
highs back to around 40 north and the mid 40s south. Another shrtwv
and cold front is expected to move through Sun night into early with
another batch of light pcpn. The GFS take a track more directly
through Lake Superior and Upper Michigan compared to the ECMWF or
GEM.

Mon-Wed, additional clipper shrtwvs are likely to move through the
region bringing additional pcpn chances. However, even through
confidence with the strength/timing is low, none are expected to be
strong enough to produce significant pcpn amounts.
&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 714 PM EDT WED APR 15 2020

Dry low levels will largely bring VFR conditions at all TAF sites
through the entire forecast period. Could be some lake effect
flurries at KIWD and KCMX mainly this evening in a westerly flow.
&&

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 340 PM EDT WED APR 15 2020

Winds will generally stay below 25 knots through the end of this
week. A surface trough will move through Lake Superior beginning
late Saturday into Sunday which may bring a period of SW winds
gusting up to 30 knots. Winds will shift NW and relax below 25 knots
again. Some models suggest a passing sfc low pressure system on
Monday, which may bring another increase in winds to 30 knots, but
confidence is relatively low for this still.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.
&&

$$

SHORT TERM...JP
LONG TERM...JLB
AVIATION...Voss
MARINE...JP