Scientific Forecaster Discussion
fxus62 kkey 161859
Area forecast discussion
National Weather Service Key West Florida
159 PM EST Sat Nov 16 2019
Florida Keys weather will feature a pleasantly dry air mass and
temperatures running a few degrees below normal through about
Wednesday night, with little or no precipitation. If we can Eek
out 6 consecutive days with highs below 80f at Key West, this
would be the longest such stretch since January 25-31 of this
year. A stretch of 5 such days was last observed at eyw from
Short term...(tonight through tuesday)
you can unmistakably feel the change in air mass over the Florida
Keys since yesterday. Strong northwest breezes have quickly
ushered in a cooler and drier air mass following a cold frontal
passage last evening. This is the first day since March 27 with
highs in the 70s over the Keys, and dewpoints are currently near
a pleasantly non-muggy 60 degrees.
A big mid-latitude low is deepening off the SC coast this
afternoon. The low will continue to produce gusty northwest
breezes over the Keys through early evening. Winds will start to
slowly ease and become more northerly late this evening as the
low center begins to slowly exit off to the northeast. In the
northwest flow, mesoscale models show a few light showers or
sprinkles spreading in from the Gulf across the Lower Keys
through this evening due to the effects of cool air sweeping
across the warm Gulf waters. This will destabilize the layer below
about 6,000 feet, which should be deep enough for a few short-
lived sprinkles or showers. The northwest cross-Gulf wind
trajectory will bring those weak showers across Lower Keys and
possibly the Middle Keys, but not the Upper Keys.
The flow across North America is fast and progressive right now.
The tail end of a trough currently exiting the northern U.S.
Rockies will race across the central U.S. And brush across the
Florida Peninsula on Monday. In its wake, surface high pressure
will fill in across the western Gulf on Monday, past the northern
Gulf Coast on Tuesday, then across Georgia and South Carolina on
Wednesday. This will maintain northwest and northerly low-level
winds through Tuesday night, becoming more northeasterly on
Wednesday. The persistent northerly component will bring continual
reinforcement of drier and somewhat cool Continental air across
the Keys through Tuesday night. As long as northwest or north flow
continues across the Gulf into the Lower Keys, this will support
scattered to occasionally broken cold-air stratocumulus clouds,
along with a couple of short-duration sprinkles or light showers.
In contrast, the Upper Keys will be downwind of the Florida
Peninsula. Without the modifying effect of the Gulf there, the
Upper Keys will remain sunnier and precip-free.
A turn to more northeasterly low-level winds on Wednesday will
more solidly clear out the Lower Keys, while starting a modifying
trend for the Upper Keys.
Long term...(thursday through next saturday)
low-level flow will become more easterly on Thursday as surface
high pressure moves off the southeast U.S. Coast. This wind flow
out of the Bahamas and across warm Gulf Stream waters will start
a period of air mass modification. The initial return of ocean-
modified air can sometimes come with a wave of low-topped showers,
and the GFS-based statistical guidance shows a small bump in pops
late Thu above 10 percent.
Agreement among global models falls apart next Friday and
Saturday. Disagreement seems rooted in how quickly to eject a
cutoff upper low over the southwest U.S. And the models' handling
of downstream features. Most of the disagreement manifests itself
in timing differences of fronts and other features moving through
the southern jet stream and across the southeast U.S.. the
European ensemble members favor a slower west-to-east progression
of southern stream features than most GFS counterparts. Our
official forecast will rely on the slower idea of the European
ensemble mean, which holds off any further frontal passages or
rain-producing systems until after day 7. This is in contrast to
the GFS ensemble mean which shows a front approaching as soon as
The bottom line for the forecast is a return to near or slightly
above normal temperatures next Friday and Saturday, with little
more than an isolated shower in moderate easterly low-level flow.
deepening low pressure southeast of the South Carolina coast this
afternoon will gradually lift out and exit to the northeast
through Sunday night, leaving behind broad and weak high pressure
centered over the western Gulf on Monday. A reinforcing cold front
will push south through the waters on Tuesday, then strong high
pressure will move by to our north across the southeast states on
Wednesday and Thursday.
So strong northwest breezes will continue this evening over the
Gulf, Lower Keys, and western straits. The pulley ridge buoy in
the southeast Gulf is currently measuring a wave height of 9
feet, owing to the long cross-Gulf fetch of strong northwest breezes.
The big low center off the southeast U.S. Coast will develop a
northerly swell that will surge southward off the Gold coast
tonight. It will start affecting the eastern straits on Sunday,
causing seas there to quickly build to 5 to 8 feet, even as winds
ease. Northerly swell will continue over the eastern straits
through at least Monday night.
VFR conditions are expected at both eyw and mth island terminals for
the rest of this afternoon and evening. While only scattered cloud
coverage is expected through 06z, instability from the relatively
cold air mass over the warm Gulf waters may result in broken coverage
at times this evening, especially at eyw, and perhaps a couple of
light showers. Post-frontal gusty winds will gradually decrease
after around 03z tonight to around 10 knots.
tidal anomalies lurched downward a bit yesterday, but the downward
trend appears to have halted today.
The middle and Upper Keys will experience another round of minor
tidal overflow late this afternoon and tonight, but the total tide
level may be a little lower than last night. The Blackwater Sound
gage just measured its lowest water level in over 5 days with the
recent low tide there. However, even that was still above our
criteria for tidal overflow.
Over the Lower Keys, the Key West tide gage showed a tidal anomaly
of 0.7 feet during the minor high tide early this afternoon. If
this anomaly continues, then the higher high tide late tonight
would fall mere hundredths of a foot shy of reaching criteria.
This does not instill much confidence in the thought of dropping
the coastal flood statement. Plus, the estofs model seems to show
a slight resurgence over the Lower Keys this evening as the
northwest winds push water across the shallower Gulfside waters
toward the Lower Keys. Will therefore extend the statement for
minor tidal overflow over the Lower Keys through tonight and
evaluate again on Sunday.
in 1943, the daily record rainfall of 5.59 inches was recorded in
Key West. Rainfall records date back to 1871.
GM...Small Craft Advisory for gmz033>035-044-054-055-074-075.
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