Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 04/16/20
Area Forecast Discussion
National Weather Service Honolulu HI
339 PM HST Wed Apr 15 2020
.SYNOPSIS...
Trade winds will continue to strengthen and stabilize tonight,
focusing typical showers over windward slopes. A stable and
moderate trade wind flow will prevail Thursday through Saturday.
Trade winds will weaken and become briefly disrupted on Sunday and
Monday, leading to a somewhat dry land and sea breeze pattern.
Trade winds may return by Tuesday.
&&
.DISCUSSION...
Trade winds are strengthening as the atmosphere becomes
increasingly stable. With a compact upper level low stalled
nearly 200 miles southeast of the Big Island continuing to
weaken, afternoon soundings and recent aircraft data confirm that
a building mid level ridge is producing increasingly stable
conditions with a low level inversion. As the strengthening trades
bring in a drier air mass, dropping dew points have decreased
humidity, and only modest showers have been observed across
windward slopes today with a few showers briefly flaring over
leeward Big Island this afternoon. Expect modest showers to be
largely confined to windward slopes tonight. The upper low is
also sending a shield of high clouds from Molokai to the Big
Island. These clouds will thin out tonight.
Stable trade wind weather will prevail Thursday through Saturday.
Trades will be at moderate strength, and as a mid level ridge
builds in, stable conditions will confine typical showers to
windward slopes.
Trades will shift southeasterly on Sunday and will likely be
disrupted on Monday as an approaching front pushes the
subtropical ridge over the islands. A stable land and sea breeze
pattern will take over by Sunday afternoon. There will be
increased cloud cover each afternoon, but we are not expecting
much rainfall.
Easterly trades may return Tuesday and Wednesday. The GFS and
ECMWF show the front stalling and dissipating north of the state
and the subtropical ridge strengthening as it retreats northward.
As a result, east to perhaps southeasterly winds will fill in
under stable conditions.
&&
.AVIATION...
Moderate trade winds will carry in a few low clouds and showers
that will focus over windward areas late tonight and early
Thursday morning. VFR conditions will prevail with the
possibility of isolated mountain obscurations/MVFR ceilings within
showers. Cirrus clouds will decrease over the eastern past of the
state this evening.
&&
.MARINE...
Surf along north and west facing shores will remain elevated through
the upcoming weekend as a series of northwest swells move through,
the first of which is expected to peak tonight. Observations from
Buoy 51101 NW of Kauai showed a spike in this swell a couple of feet
above model guidance for a few hours this morning. This should be
enough to boost surf heights to low-end warning level for north and
west facing shores of most islands tonight, so a High Surf Warning
has been issue for those areas. The warning-level surf is expected
to be short-lived, however, with surf returning to advisory level
Thursday, then continuing to lower through Friday. With the higher
than expected swell heights, enough energy will make it into west
facing shores of the Big Island tonight into Thursday to warrant
a High Surf Advisory there tonight through Thursday.
A Small Craft Advisory is in effect for most Hawaiian waters due
to either the large northwest swell or strong winds around Maui
County and the Big Island through tonight. Then, the SCA will be
scaled back to just the typically windy areas around Maui and the
Big Island and extended through the day Thursday. Another northwest
swell will build Friday night into the weekend for another round
of advisory level surf as it peaks Saturday night, then slowly
eases Sunday into Monday.
Surf along south facing shores will remain small through the week.
However, a long-period south swell will affect Hawaii by next Monday.
Surf heights could exceed advisory level as it peaks early next week.
Surf along east facing shores will trend upward during the second
half of this week as trade winds strengthen.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Thursday for for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai, and for north
facing shores of Maui.
High Surf Advisory from midnight tonight to 6 PM HST Thursday
for Kona.
Small Craft Advisory until 6 PM HST Thursday for Pailolo Channel-
Maalaea Bay-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-Big
Island Windward Waters.
&&
$$
DISCUSSION...Wroe
AVIATION...Foster
MARINE...TS
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
845 PM CDT Wed Apr 15 2020
.UPDATE...
845 PM CDT
Evening update...
Going forecast for tonight appears on track, with no significant
changes needed at this time. Diurnal cooling with sunset has
allowed winds to diminish and stratocu to decrease in coverage.
Made some minor adjustments to digital forecast grids based on
obs trends, but otherwise forecast looks to be in good shape.
Not much new model data to look at with respect to tomorrow
night`s snow system. 18Z runs of the ECMWF and GFS are similar to
their previous versions, with perhaps a slightly slower timing.
NAM and it`s high- res derivatives remain farthest north with
heaviest precipitation axis. System looks to be mainly driven by
ageostrophic forcing associated with strong frontogenetic forcing
and upper level jet divergence with instability present, likely
resulting in a narrow swath of potentially moderate-heavy
snowfall. Full suite of 00Z guidance should help narrow down
details somewhat overnight.
Ratzer
&&
.SHORT TERM...
323 PM CDT
Through Thursday night...
With this morning`s snow having already mostly melted across the
area, temperatures have risen back into the upper 30s and low 40s.
Generally quiet weather is expected tonight through Thursday
afternoon aside from some lingering light flurries possible along
the lake this evening. Below normal temps continue tonight with lows
in the 20s.
The biggest concern in the short term is the potential for another
round of snow late Thursday into early Friday morning. Ingredients
continue to come together for a narrow band of very heavy snow to
set up somewhere in our forecast area, currently favored across
areas south of I-80.
Looking at the big picture, an elongated trough and broad cyclonic
flow is in place across much of the CONUS with an embedded
shortwave/mid-level trough feature beginning to translate southeast
out of the northern Rockies. Lee cyclogenesis is expected by later
this evening which then tracks east northeast along a well defined
baroclinic zone through the day tomorrow toward the Great Lakes
region.
In addition to upper level jet support with pseudo-jet coupling, our
area is under the favorable right entrance region for lift. Mid
level support is also quite favorable for heavy snow potential. A
strong low level jet in place and warm air advection, with strong
isentropic upglide will bring in plenty of moisture into the area.
This combined with very impressive omega/ascent in the dendritic
growth zone could allow for better/more efficient snowfall
(snow:liquid ratios) than would otherwise be expected this far out
of season.
In addition, forecast thermal profiles show a rather deep isothermal
layer around -5C, which given the strong omega through that layer
and extending well above that layer is quite favorable for the
formation of large aggregate snowflakes that tend to be fast to
accumulate, even in marginal sfc temps. In addition, cross sections
show a well defined FGEN circulation north of the surface boundary
which suggests moderate to locally heavy snow is possible in a
narrow band, particularly given the deep layer of strongly negative
EPV, suggesting potential for either upright or slantwise
convection. Strong instability with intense FGEN typically results
in narrow heavy precip bands, with sharp cut offs to the north and
south of the band.
The NAM model suite appears too far north/warm with the ECMWF and
GFS having been fairly consistent with placement of the most snow,
so am leaning toward this solution, in alignment with WPC. Thus have
trended a bit higher in snowfall totals with this latest forecast
package. As mentioned earlier, it is certainly possible we see even
higher totals in a narrow band.
Temps will be quite warm at the start of the event (Upper 30s low
40s). Although there may be a brief transition from rain to snow
Thursday afternoon, once the precipitation begins, expect
temperatures to quickly drop to the wet-bulb temp and hover at or
just above freezing through the majority of the event allowing for a
quick change over to all snow.
Uncertainties: The big question still remains where this narrow
heavy snow band will ultimately set up and how wide the coverage is.
It is certainly possible somewhere in the forecast area sees double
digit snowfall totals but it is too soon to identify where that may
be. In these heavy banded snow scenarios, the cutoff can be quite
sharp, with areas less than a county away might not see much more
than a light dusting. With this in mind, current snowfall totals are
more broad-brushed to account for this uncertainty. It is likely this
will continue to narrow as confidence increases. Stay tuned to
future forecasts as we continue to refine the placement of this band.
Given these uncertainties, primarily in the exact placement of the
band, have decided to hold off on the issuance of a watch, and will
let later shifts re-evaluate as more models come in and the event
gets closer.
Petr/Izzi
&&
.LONG TERM...
217 PM CDT
Friday night through Wednesday...
Following the early Friday snow event across the area temperatures
will begin to rebound back into the 50s on Saturday and Sunday.
This occurs as the flow quickly increases and becomes gusty out
of the southwest on Saturday. While much of the weekend should be
dry across the area, I can`t rule out the potential for a period
of light showers Saturday night into Sunday as a weakening frontal
boundary and upper trough shifts over the area.
Weather conditions look to remain mainly dry early next week
before better chances for rain return to the forecast for the
middle part of the week. Temperatures look to be seasonably mild,
in the low to mid 60s during this period.
KJB
&&
.AVIATION...
For the 00Z TAFs...
Aviation forecast concerns:
* Breezy west-northwest winds gradually diminishing this evening.
* Snow developing into the area mid-late Thursday evening. Snow
may become moderate/heavy just beyond the current end of ORD/MDW
30 hour forecast, though uncertainty remains in exact track of
storm.
Early evening surface analysis places an elongated high pressure
ridge from the Dakotas into central IL. North of the ridge axis,
west-northwest winds persist, with deep mixing to nearly 9000 ft
agl indicated in recent AMDAR soundings. While the surface
pressure gradient is not particularly tight, the depth of the
mixing has been enough to produce sporadic gusts in the 15-20 kt
range, and forecast soundings support this potential for another
hour or two this evening. Broken high-based stratocu deck will
likely help keep temps from cooling off to quickly, slowing
decoupling of the boundary layer. However, winds should gradually
diminish with gusts becoming less frequent with diurnal cooling.
Surface ridge weakens Thursday, as pressure falls develop in
association with a compact but strong disturbance approaching
from the Plains. Initially, this will yield a gradual increase and
lowering in mid/high VFR cloud cover especially during the
afternoon, and modest west-northwest winds backing southwest.
Clouds continue to lower and thicken Thursday evening, with snow
expected to overspread the area from the southwest during the mid-
late evening hours. While there could briefly be a mix of
rain/snow at onset, dry air in the lower portion of the thermal
column should support a fairly quick change to all snow as precip
intensity picks up. Some differences remain in guidance with the
exact onset time, generally in the 03-05Z time range, but once it
develops snow should be prevalent beyond the end of the 30 hour
ORD/MDW TAFs. Snow could potentially be heavy at times off to the
south of the terminals, though uncertainty remains as to the exact
track of the heavier snow axis. A period of IFR/LIFR conditions
is not out of the question for Chicago area terminals in
accumulating snow from after midnight Thursday night into Friday
morning, before the system quickly moves off to the east.
Ratzer
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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