Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 02/08/18
Area Forecast Discussion...Updated
National Weather Service Billings MT
859 PM MST Wed Feb 7 2018
Updated forecast to increase precipitation chances over southeast
Montana and lower temperatures a few degrees. RUC is delaying
windshift to the southwest in Billings until after midnight so mix
of freezing rain and sleet showers possible until temperatures
warm towards daybreak. borsum
.SHORT TERM...valid for Thu and Fri...
Active and complex weather pattern is anticipated through the
beginning of the weekend. The sensible weather will revolve
heavily around where the arctic boundary sets up and when it
finally makes a strong push through Montana and into the mountains
Thursday and Thursday night.
As 3 pm the arctic boundary was located along a line from Ingomar
southeast into Broadus and into western South Dakota.
Precipitation was beginning to fill in to the east of this
boundary as a disturbance slides in overhead. The precipitation
type is going to be tricky through the next 12 to 24 hours, but
generally along and just to the east of this boundary folks should
be ready for all kinds of winter precipitation such as
rain/freezing-rain/sleet/snow. When you get will depend how far
you are to the east the the boundary edge. Currently expecting
this boundary to slide west of its current position tonight.
Forecasting of these arctics is difficult but it appears it will
set up somewhere in a line between Harlowton/Roundup,
Billings/Forsyth, Hardin/Colstrip, and Sheridan/Broadus. West of
these areas folks should expect rain in any precipitation falling
tonight. A mix of freezing rain/sleet/light-snow is expected for
folks between and just of these cities. Finally, folks in locations
like Forsyth, Baker, Miles City, and Ekalaka should expect all
snow tonight. The best chance for the freezing rain will be
between 9pm tonight and 9am Thursday morning before the arctic
begins to make a steady push through the region on Thursday
Biggest changes to the forecast today involved lowering snowfall
totals over much of the region (especially in western zones like
Billings, Columbus, and Big Timber) as the warm downslope flow
will keep a lot of the precipitation as rain initially in these
areas on Thursday. However, snowfall totals will still cause
quite a headache, especially east of Billings where the cold air
already in place will allow the precipitation to be all snow.
Still expecting at least 4 to 6 inches in Billings/Roundup and
locations west while east of Billings we`re expecting at least 6
to 8 inches. The heaviest snow will fall Thursday and Thursday
afternoon in locations east and north Billings/Roundup/Sheridan
while locations in and west of these cities will see the heaviest
snow behind the arctic front Thursday evening and early Friday
Much colder temperatures move in Friday afternoon with highs in
the low teens and single digits. Snow will gradually end in the
northeast zones near Miles City and Baker by the afternoon, but
expect the snow to continue through at least Friday evening for
Billings/Sheridan/Roundup and other zones west of these locations.
A cold night is in store Saturday morning with nearly the entire
area seeing below zero temperatures. Wind chills will drop into
the negative 20s and 30s in eastern Montana Saturday morning so
bundle up. Dobbs
.LONG TERM...valid for Sat...Sun...Mon...Tue...Wed...
Pattern will remain active in strong northwest flow aloft. The
period will start with a drying trend on Saturday, but a Klondike
chinook kicking in by Saturday night. Temperatures on Saturday
will be chilly with fresh and deep snow cover. As winds increase
Saturday night, the arctic will get driven east, allowing
readings to moderate into the 30s. Temperatures may be too warm
for Saturday with deep snow cover, so lowered highs a couple of
degrees. Winds will increase over the western foothills Saturday
night and Sunday. This could produce some blowing snow over the
western foothills, especially Saturday night. Will need to watch
The next system of note is set to arrive Monday and Monday night.
Models are in pretty good agreement in merging an upper low from
off the BC coast to an upper trough dropping southeast from the
northern territories. This will drag the arctic back in and set up
another snow event. The ECMWF was faster and not quite as cold
with this system, compared to the GFS. The GFS was also quite a
bit wetter. Have raised PoPs for this event as the GFS has been
doing better compared to the ECMWF on system arrival time and
intensity, of late.
Northwest flow will remain through mid week with another wave
arriving on Wednesday. Some indications are that the pattern will
switch a bit with a trough replacing the ridge along the western
coast, putting the forecast area in zonal flow. This would be for
late next week and is a long way out there. TWH
A wintry mix of precipitation will be over the area overnight.
Expect snow E of KBIL with MVFR/IFR conditions. There will be a
chance of freezing rain, freezing drizzle, sleet and snow over the
KBIL area with LIFR conditions. Elsewhere, expect rain and snow
showers with VFR/MVFR conditions. There will be areas of mountain
obscuration with widespread obscurations over the
Beartooths/Absarokas. On Thursday, good confidence that rain and
snow will turn to all snow late in the afternoon through the
evening as an arctic front pushes W through the area. Conditions
will deteriorate to IFR/LIFR. Expect LLWS over KSHR in the
morning. The mountains will be obscured. WSW surface winds in KLVM
will increase in speed, with 50 kt gusts by midday Thursday. Arthur
.PRELIMINARY POINT TEMP/POPS...
Thu Fri Sat Sun Mon Tue Wed
BIL 026/034 901/006 906/016 007/033 014/029 007/030 016/036
37/W 86/J 42/J 00/N 24/J 31/B 01/B
LVM 040/050 008/019 001/023 010/038 020/034 011/034 021/039
36/W 86/J 52/J 00/N 24/J 31/B 12/J
HDN 021/028 902/008 910/013 000/031 009/027 003/027 010/035
68/J 85/J 42/J 00/U 24/J 31/B 01/B
MLS 006/011 909/003 913/007 903/023 004/020 902/020 007/029
99/J 52/J 11/B 00/U 23/J 21/B 01/B
4BQ 018/031 903/007 909/010 901/029 010/027 003/025 010/034
9+/J 74/J 21/B 00/U 23/J 31/B 01/B
BHK 003/008 911/003 913/008 903/023 005/022 902/021 007/031
9+/J 52/J 10/B 00/U 23/J 21/B 01/B
SHR 026/041 000/008 905/015 902/033 012/030 006/029 012/036
36/W 86/J 52/J 00/U 24/J 31/B 01/B
MT...Winter Storm Watch in effect from Thursday afternoon through
Friday afternoon FOR ZONES 28-34-39-41.
Winter Weather Advisory in effect until 11 PM MST Friday FOR
Winter Storm Warning in effect until 11 PM MST Friday FOR
Winter Storm Watch in effect from Thursday evening through
Friday afternoon FOR ZONES 40-56-64>66.
Wind Advisory in effect until 6 PM MST Thursday FOR ZONES
Winter Storm Warning in effect until 5 PM MST Friday FOR
WY...Winter Storm Warning in effect until 5 PM MST Friday FOR ZONE
Area Forecast Discussion
National Weather Service Columbia SC
943 PM EST Wed Feb 7 2018
The cold front will move through the area this evening with high
pressure ridging into the area from the north Thursday.
Moisture will increase again Friday ahead of a warm front. The
front and associated moisture will be in the area Friday night
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Cold front continues moving across SC and will move into the
western Midlands over the next couple of hours and the remainder
of the forecast area early tonight. WSR-88D returns continue
showing scattered showers over much of the Midlands with the
heaviest rain along the coast. Over the next several hours the
remaining showers will move eastward and end with frontal
passage overnight. Toward daybreak clouds will begin clearing as
drier air begins moving into the region. Winds have diminished
as the heaviest cells have moved east of the area so the lake
wind advisory has been cancelled. Overnight lows will range from
the low 40s in the northern and western Midlands to the low 50s
in the southern CSRA.
.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
Dry weather as transitional high pressure will move over the
area Thursday and begin to move offshore Thursday night.
Temperatures will be close to seasonal levels.
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
The latest GFS shows a moisture increase Friday with an onshore
flow and approaching warm front. The warm front may linger
Saturday ahead of a cold front approaching from the west. The
GFS shows this slow moving cold front lingering in the region
into the early portions of next week, which will keep conditions
unsettled into Wednesday. Temperatures will be generally close
to or a few degrees normal for the majority of the longer term.
.AVIATION /03Z THURSDAY THROUGH MONDAY/...
Showers along the cold front will diminish tonight. The HRRR
indicated showers will be east of the terminals by 08z.
Stability has increased behind the main band of showers and wind
has diminished. Also, the main low-level jet has shifted
northeast of the area diminishing wind shear. Expect veering
west to north wind mainly 5 to 10 knots as indicated by the GFS
LAMP during the gradual frontal passage tonight. Observation
trends and the GFS LAMP support mainly MVFR ceilings through
much of the night. Boundary layer wind should limit fog. Drying
well behind the cold front should dominate after 12z and we have
forecasted VFR conditions. The GFS and NAM MOS support
northeast wind 5 to 10 knots.
EXTENDED AVIATION OUTLOOK...No impacts to aviation expected
Thursday and Friday. Rain and associated restrictions may occur
Friday night through Monday associated with a slow moving
frontal system in the region.
Area Forecast Discussion
National Weather Service Grand Rapids MI
933 PM EST Wed Feb 7 2018
Issued at 330 PM EST Wed Feb 7 2018
Lake effect snow will diminish this evening over the lake shore
areas. It will remain quiet the rest of the night and through
Thursday. But by Thursday night a long duration snow is expected to
begin. It is expected to last 24 to 30 hours, into Friday night.
The heaviest snow is expected to fall south of I-96, where 6 to 10
inches will be possible. 4 to 8 inches will be possible along the I-
96 corridor. Lighter snow is expected farther north, with 2 to 3
inches expected toward Highway 10. Stay tune for further updates on
More snow will be possible Saturday night and Sunday, and again
Wednesday. It will remain cold through the next 7 days with daily
high temperatures in the 20s to around 30.
Issued at 931 PM EST Wed Feb 7 2018
I have significantly increased POPs over our western CWA from now
through 7 pm tomorrow. This is based on both radar data and the
lift in the 925 to 850 layer as shown by numerous runs of the
HRRR, HRRRX, RAP and NAM models. All of these models show lift in
that layer west of US-131 through 7 pm Thursday. That layer is
also saturated and well within the DGZ temperature range. So while
the inversion heights are shallow, mostly near 5000 ft agl., we
will still get snow showers. In fact our 88d snow accumulation
algorithm shows 1 to 2 inches already in west central Allegan
County between 5 pm and 9 pm. That area of best lift is southwest
of GRR this evening but becomes solid north to south by 7 am
Thursday morning from US-131 to US-31 from South Haven to
Ludington. My thinking is it will snow through the night and all
day Thursday in our western row of counties. Snowfall will be
limited due the shallow nature of the clouds but I can see an inch
two more falling in some areas by late Thursday afternoon. The Hi
Res NMM does the best job of forecasting the snowfall pattern so
I used that to model my pop and precipitation amounts.
.SHORT TERM...(This evening through Friday night)
Issued at 330 PM EST Wed Feb 7 2018
Will allow the current lake effect headline to end. Will hoist a
Winter Storm Watch for the Thursday night to Friday night period.
The evening lake effect snow showers will still make for slippery
travel, but it will be winding down by early to mid evening. Most
areas will see an additional inch or less as surface ridging takes
over by late evening.
The main story will be the developing long duration storm that moves
in toward midnight Thursday night. A sharpening surface boundary
will develop near the MI/IN border. Isentropic upglide will
increase as this boundary tightens and snow will develop north of
the boundary by late evening. The snowfall will get a boost by
daybreak Friday as the right entrance region of the upper jet comes
overhead. Also seeing some FGEN develop in this time frame that
lingers into the afternoon, especially south of I-96. The FGEN
wanes by Friday evening, but upper jet doesn`t move off until late
evening. Meanwhile at the surface, waves of low pressure ripples
along the boundary by Friday evening. These gradually cause the
boundary to sag slowly southeast. With the boundary and upper jet
moving farther away, the snow should ease up into Friday evening.
So all told this will be a 24 to 30 hour event with rather steady
snowfall throughout and occasionally heavy bursts. The snowfall will
be the heaviest south of I-96 where FGEN is most likely, along with
the other forces. Areas between I-96 and Highway 10 should be in the
4 to 7 inch range, and slightly less north of Highway 10. The
amount of snow south of I-96 could bring commence to halt, including
closings and possible road closures. The areas between I-96 and
Highway 10 will see moderate impacts, with closing also likely. One
good factor to this event will be that the winds will remain light,
generally 5 to 10 mph. Initially from the ESE Thursday evening,
gradually turning to NNE by Friday evening.
.LONG TERM...(Saturday through Wednesday)
Issued at 330 PM EST Wed Feb 7 2018
The main challenge in the long term deals with the potential for
impacts over the weekend.
To start...a mid level trough will be approaching from the plains
Saturday. As this feature nears...the moisture deepens up and the
lift increases. An area of snow is expected to spread in from the
west southwest during the day. The risk for snow will then persist
into the night. The snow could come in more than one waves. Models
are in relative agreement on this scenario. Accumulations look
possible. As for impacts...they look to be light or possibly
moderate as some slick roads are possible. Today we saw temperatures
around 20 degrees with snow. Roads temperatures did climb into the
30s during the day. So with Saturday a few degrees higher...daytime
impacts may be limited.
The pattern changes once this trough goes by for early next week.
The mid level flow becomes more zonal. This will act to shift the
arctic airmass into Ontario and Quebec. At the surface...warm air
advection develops here in the Great Lakes Region. Temperature
values look to actually go near to above normal by Tuesday. Perhaps
warm enough for some of the precipitation to be rain.
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 652 PM EST Wed Feb 7 2018
Just about all of our TAF sites have had IFR conditions in snow
showers during the past hour. As drier air moves in from the west
the snow showers are expected to diminish. Normally at night the
snow showers only impact the lake shore, in this case that would
mean the MKG taf site. However tonight due to winds becoming more
westerly, only the I-69 taf sites should remain snow free after
03z and also be VFR with ceilings. Most of the taf sites should be
VFR by 18z Thursday.
Issued at 330 PM EST Wed Feb 7 2018
Current ice jams and potential for new ice jams to form will be the
main hydrologic concern through the weekend. Cold temperatures will
be favorable for new ice formation on area rivers, and will not
favor the breakup of current ice jams. The next round of widespread
snow is expected Thursday night and Friday, with moderate to even
heavy amounts possible along and south of I-96. Water from this snow
will remain locked in the snowpack and not contribute to runoff.
Warmer temperatures are expected next week, with highs possibly
rising above freezing Tuesday and Wednesday. A wintry mix of
precipitation will also be possible Tuesday into Tuesday night. A
small amount of snowmelt and liquid precipitation may create some
runoff into area rivers. The combination of runoff and warmer
temperatures could allow river ice to begin shifting.
Flood Advisories will remain in effect for the Grand River near
Portland and the Looking Glass River near Eagle until water levels
fall below bankfull and stabilize. Minor flooding of low-lying areas
has been reported in each location due to ice jams. Water levels in
areas near ice jams will continue to fluctuate by as much as three
feet until ice breaks free. Please see the latest Flood Advisory
Statement for more detail.
MI...Winter Storm Watch from Thursday evening through late Friday
night for MIZ064>067-071>074.
Area Forecast Discussion
National Weather Service Memphis TN
922 PM CST Wed Feb 7 2018
Forecast is in good shape no significant changes.
A distinct clearing line is slowly advancing across the Midsouth,
oriented along a line from around Paris, TN to Tunica, MS.
Clearing skies and light north winds will allow temperatures to
cool into the 20s across most of the Midsouth...even the low 20s
across Northern areas. Temperatures will trend warmer tomorrow and
Friday with rain returning to the Midsouth late Friday.
.PREV DISCUSSION... /issued 238 AM CST Wed Feb 7 2018/
A cold front has just passed through Monroe County, MS. As a
result, winds have turned around to the north or northwest across
the entire Mid-South. Colder temperatures are filtering into the
area. The freezing line currently stretches from near Harrisburg,
Arkansas northeastward to Union City, TN. North and west of this
line, freezing rain or freezing drizzle is occurring while south
and east of this line remains rain. Latest HRRR shows
precipitation quickly shifting east over the next 2-4 hours. May
be allowed to cancel at least the Ice Storm Warning early. Will
make a last minute call just before 4 AM.
Precipitation should be completely out of the Mid-South by
18Z(12 PM CST.) Clouds will likely be slower to clear.
Temperatures will remain chilly today with highs ranging from the
mid 30s across the Missouri Bootheel and Northwest Tennessee to
around 50 degrees near Monroe County, MS. However, highs in
locations southeast of Tupelo, MS will occur at 12Z(6 AM CST).
Expect temperatures to drop into the lower to mid 40s over the
course of the day.
Clouds will clear by tonight as SFC high pressure builds into the
region. With clear skies and light winds, expect temperatures to
plummet into the lower to mid 20s for lows.
Temperatures will begin to rebound on Thursday as the high shifts
east. Expect highs to range from the mid 40s to the mid 50s.
The warming trend will continue into Friday. Although, chances for
rain will begin to occur by Friday afternoon as a low pressure
system begins to develop near the Texas Coast and a secondary one
develops across the Southern Plains. Chances for rain will
increase Friday Night into Saturday. Once the models get beyond
Saturday, solutions begin to diverge on exactly how the Southern
Plains low pressure system will evolve. The ECMWF evolves the
system similar to the low pressure system that tracked through the
Mid-South last night. Thus, changing rain over to freezing rain
late Saturday Night into Sunday morning. The GFS is warmer with
the system keeping precipitation rain. Meanwhile, the Canadian
ends precipitation before temperatures drop below freezing. For
now will keep precipitation all liquid until there is some model
Models bring another system into area by Monday Night into Tuesday
thus keeping rain chances in the forecast and little drying time
MVFR deck will continue to influence the TAF sites early in the
period...with improvement to VFR expected later tonight. North to
northeast winds of 9-13kts...will diminish slightly and veer more
Area Forecast Discussion
National Weather Service San Francisco Bay Area
340 PM PST Wed Feb 7 2018
.SYNOPSIS...A strong ridge of high pressure over the eastern
Pacific will remain in place through late week resulting in dry
weather conditions and temperatures well above seasonal averages.
.DISCUSSION...as of 01:28 PM PST Wednesday...Unseasonably warm
and dry conditions prevail region- wide as a strong ridge of high
pressure remains parked over the eastern Pacific. This pattern is
likely to persist through at least late week with daytime
temperatures a good 10 to 20 degrees above seasonal averages.
Overnight conditions will remain clear with decent cooling in the
valleys, yet temperatures will still be warmer than what`s typical
this time of year.
In the short-term, an area of stratus and fog can be see on visible
satellite imagery off of the Big Sur coast as a southern surge
pushes northward. The latest HRRR is the only model picking up on
this and shows much of this staying offshore and not impacting the
urban areas of the Monterey Peninsula through this evening. Thus,
have added cloud cover and patchy fog to the forecast for the
southern coastal waters. Will continue to monitor this through the
afternoon and evening.
The medium range models continue to show the dominate ridge
weakening and retreating to the west at some point this weekend
which would allow for slight cooling aloft. However, this was
previously forecast to occur as early as Friday and is not trending
to be later in the weekend. Thus, have kept temperatures warmer for
Friday and Saturday with the cooling trend likely later in the
weekend into early next week. It is at this point when the models
show a mid/upper level trough dropping southward into the Pacific
Northwest and then through California and the southwestern portion
of the country. At this time, do not see any significant signs of
precipitation across our region from this system. Thus, will
maintain dry weather conditions through the forecast period. Even
with the slight cooling trend expected, temperatures are likely to
remain above seasonal averages into early next week.
.AVIATION...As of 3:40 PM PST Wednesday...For 00z Tafs. VFR
conditions through the period expected. Southerly surge of stratus
is now making its way north of the Monterey Peninsula and into
the bay. Short term models as of now are not expecting much if any
impact to coastal airports. Winds generally light through the
Vicinity of KSFO...VFR. Clear skies and Light winds. As of now, no
impact to KSFO is expected from the southerly surge of stratus.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR with generally light winds. The
southerly surge of stratus is currently wrapping into the bay, but
mainly confining itself to the water and immediate coastal areas.
There is a chance over the next few hours the stratus could reach
KMRY. If that happens, not expecting cigs to go BKN although could
be SCT at a few hundred feet.
Here are the record high temperatures for Wednesday, February 7,
2018 through Friday, February 9, 2018.
Location Wednesday Thursday Friday
SF Bay Area
Healdsburg 80 in 1930 83 in 1930 79 in 1907
Santa Rosa 74 in 2006 74 in 2006 75 in 2006
Calistoga 85 in 1917 85 in 1917 79 in 2016
Kentfield 71 in 2006 74 in 2016 74 in 1945
San Rafael 74 in 2016 74 in 2016 70 in 2016
Napa 79 in 2011 78 in 2016 75 in 2006
San Francisco 73 in 2006 75 in 2016 73 in 2006
SFO Airport 73 in 1987 73 in 2016 71 in 2016
Half Moon Bay 75 in 1987 77 in 1987 77 in 2016
Oakland Airport 74 in 2016 79 in 2016 70 in 2016
Richmond 74 in 2015 78 in 2011 75 in 1988
Livermore 78 in 1917 78 in 1917 77 in 1917
Moffett Field 73 in 1987 75 in 2016 74 in 2006
San Jose 76 in 2016 78 in 2016 76 in 2016
Gilroy 76 in 2011 78 in 1963 78 in 2012
Monterey Bay Area
Santa Cruz 85 in 1954 85 in 2016 78 in 2006
Salinas 82 in 1987 83 in 2016 84 in 2006
Salinas Airport 84 in 1954 83 in 1954 81 in 2016
Monterey 85 in 1954 84 in 2016 85 in 2016
King City 85 in 1954 84 in 2006 86 in 2006
.MARINE...as of 01:24 PM PST Wednesday...A southerly wind
reversal of low clouds and fog is working northward along the Big
Sur coast late this afternoon and will continue to spread towards
Monterey Bay this evening. Otherwise light wind and seas will
prevail through Thursday under high pressure across the coastal
waters. Northwest winds will increase later Friday into Friday
night as a strong pressure gradient sets up near Cape Mendocino
with strong winds into the weekend. A large nw swell train may
arrive by Monday after a weekend of fresh wind waves.
PUBLIC FORECAST: RGass
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Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
707 PM EST Wed Feb 7 2018
.UPDATE (Overnight through Thursday)...
00Z water vapor and H4 RAP analysis shows an upper level
pattern consisting of broad ridging over the western half of
the nation and broad troughing covering the eastern states.
The Florida peninsula resides generally south of this trough
on the northern periphery of Caribbean upper ridging. One
shortwave trough exiting the southern MS valley around the
base of the eastern CONUS trough will be shearing out as it
migrates eastward overnight...but is forecast by ensemble
guidance to provide the atmospheric push to propel a weak
cold front down into the northern/central peninsula by
This front is currently located at the surface back toward
the Florida peninsula and north-central Gulf of
Mexico...and has been supporting widespread showers/storms
along the northern Gulf coast through the day. However, as
the front moves south later tonight and Thursday
morning...it will drop south of the best large scale forcing
for ascent...and also into a region of much shallower
column moisture. With all these factors...agree with the NWP
ensemble guidance which show the showers becoming much less
organized by the time they reach our far northern Nature
Coast zones...and likely be completely gone before the
frontal forcing reaches the I-4 corridor of the state.
So...current forecast does bring a few scattered showers
into the Nature coast overnight/Thursday morning...but
certainly not a washout (like some recent systems), and
rainfall accumulations will be very light. South of
I-4...doubt we will see anything in terms of shower activity
from the front. Temperatures the next 24 hours will remain
well above average for early February. Even behind this
front...there is not much in the cooler air...and locations
south of I-4...will barely notice its passage. Low temps
overnight ranging from around 60 up toward Chiefland...to
the middle 60s across central and southern portions of the
region. High temperatures Thursday ranging from the lower
70s behind the front up toward Chiefland...to the
middle/upper 70s through the I-4 corridor...and lower 80s
The greatest forecast dilemma the next 12-18 hours will be
the potential development and evolution of sea-fog. I would
love to say sea fog is an easily identifiable event and easy
to forecast. The fact of the matter is that sea fog is
extremely difficult to forecast in detail. There certainly
are good indicators of sea fog potential...but everything at
the surface and above the surface have to play perfectly
together to develop the fog...and then have to remain in
alignment to keep the fog going. Very subtle changes and
differences over time can easily bust a forecast. To make
matters worse...NWP is highly biased toward being aggressive
with its development and many models give numerous false
alarm forecast during a season. Therefore...often we have to
go off knowledge of past experience and simply give this
fog forecast a "best shot". Since the economic impacts to
shipping are so high with any sea-fog event...especially in
and around Tampa Bay...we often have to lean toward a more
aggressive forecast...and doing that overnight/Thursday
We are not going to see any fog development until the sea-
breeze circulation has completely dissolved. The sea-breeze
results in a highly mixed environment and would prevent the
stability needed for start the sea fog process.
Therefore...we got a few more hours before the potential
goes up. Once we get closer to midnight and beyond...fog
potential over the coastal waters and developing onshore
goes up...especially to the north of Venice/Sarasota where
SSTs are coolest and the SST/dewpoint depression is
greatest. One of the concerning factors later tonight is the
fact that as the front starts to approach, we end up in a
col region of the surface pressure gradient. Within this col
region winds will be very light. Contrary to radiational
land based fog...where light/calm winds are required...sea
fog actually needs some air movement to form. We almost
certainly have the dewpoint depressions in place across the
northern half of the area...but it is very border-line
whether or not we have enough air motion to actually get the
fog to develop. However...for now...lets assume it does.
Would anticipate low visibility moving onshore from around
Sarasota...up through portions of Tampa Bay and then along
the Nature Coast. These areas run the greatest risk of
significant visibility issues during the Thursday morning
commute. Once the sun comes up...diurnal mixing would slowly
erode the sea fog back toward the coast...with a
combination of mixing and frontal passage ending the threat
for most location by the second half of the day. Will be
closely monitoring satellite and model data through the
evening and update the forecast as it relates to temporal
and spatial low visibility as necessary.
.AVIATION (08/00Z through 09/00Z)...
When sea fog potential is in the forecast...the forecast for
terminals near the coast is always a difficult one. With sea
fog being such a subtle phenomena, the range of
possibilities and timing of visibility restrictions is wide.
Therefore, tried not be get too detailed just yet. Will not
see any sea fog at the coast until the sea-breeze has
completely dissolved. Therefore...bring visibility down
around midnight...with IFR conditions for the late overnight
hours and first couple daylight hours of Thursday. A gradual
improvement in conditions then begins with daytime
heating...with any fog working back toward the coast and
eventually back out over the water. Will monitor satellite
and model trends through the evening and update this
discussion as necessary.
.Prev Discussion... /issued 235 PM EST Wed Feb 7 2018/
SHORT TERM (Tonight-Thursday)...
Deep Hudson Bay U/L vortex is creating extensive cyclonic
circulation over much of North America with a L/W trough
axis through the central U.S. An U/L disturbance was
rotating through the base of the trough and will quickly
eject and lift quickly northeast overnight. An associated
area of low pressure will move rapidly from the central
Appalachians through the northeast tonight, with a trailing
cold front pushing into north Florida. However, U/L support
will be moving away from the region which will cause shower
activity along the boundary to dissipate as it sinks
gradually south over north Florida...with only a slight
chance of a shower over the nature coast. Low level moisture
will pool ahead of the front which will aid fog development
tonight across west central and southwest Florida, and fog
may be locally dense. Dew points in the mid 60s will also
create favorable conditions for sea fog to develop after
sunset. Main inhibiting factor will be the weakening
gradient over the water, which may cause mixing to become
too weak for sea fog formation.
The frontal boundary will stall across the central Florida
peninsula on Thursday as it becomes parallel to the U/L
flow. Main impact will be slightly drier air advecting
across portions of the region with dew points on Thursday
dropping into the mid 50s north to the lower 60s central.
A strong subtropical U/L ridge was centered over the
Bahamas and extended across the Gulf of Mexico and the
Florida peninsula. Strong large scale subsidence associated
with the ridge will allow temperatures to remain above
LONG TERM (Thursday Night-Wednesday)... General troughiness
over the country while weak upper ridge from the NE
Caribbean nudges east of the state through the weekend with
SW flow developing aloft bringing some energy off the Gulf
through mid next week.
Frontal boundary washes out over the area to start the
period with high pressure north of the region moving into
the W Atlantic this weekend to keep SE-S low level flow over
the FL Peninsula this to end the weekend and into next
week. Decent mean layer moisture and moving over the region
to combine with diurnal heating, bay/sea breezes to keep
isolated to scattered rain chances in the forecast. Expect a
warm, muggy, and somewhat unsettled pattern as temperatures
will remain above normal through the period.
Potential for sea fog developing tonight, otherwise rather
benign conditions next several days with no other hazards
No fire weather hazards are expected next couple of days as
the relative humidity is expected to remain above critical
levels each day.
.Preliminary Point Temps/PoPs...
TPA 66 78 63 79 / 10 20 20 10
FMY 67 83 65 82 / 10 20 20 20
GIF 65 80 63 81 / 10 20 10 20
SRQ 64 78 63 78 / 10 10 20 10
BKV 62 78 60 80 / 10 20 10 10
SPG 65 78 63 78 / 10 20 20 10