Area Forecast Discussion
Issued by NWS Melbourne, FL
FXUS62 KMLB 210907
Area Forecast Discussion
National Weather Service Melbourne FL
407 AM EST Tue Feb 21 2017
...Mid week rain/storms likely as a low pressure system approaches
from the Gulf of Mexico...
An H100-H70 ridge axis blanketing the ern seaboard will be the
primary WX feature, but will gradually lose its grip as a high amp
short wave trof over the Arklatex/NW GOMex works its way into the
Deep South/NE GOMex. Position of the ridge axis will generate a
steady E/SE flow which will tap a dry/stable airmass overhead...evng
RAOBs measured PWat values in the 0.75"-1.00" range as well as a
sharp H85-H70 subsidence inversion. Latest RAP analysis showing
regional PWats arnd 0.75" spanning the Bahama Bank/ern GOMex region
with H85-H70 lapse rates AOB 3.5C/KM and as low as 1.0C/KM. An H50
thermal ridge running the spine of the FL peninsula will put the
final nail in the coffin for any precip chances.
Patchy/shallow morning fog will burn off by 14Z. Increasing high
clouds afterward as convection over the GOMex assocd with the short
wave generates a dense CI shield. Steady onshore flow for the past
24-36hrs has fully modified the low lvls. This should offset the
cooling effect of the increasing cloud cover, allowing max temps to
warm into the U70/L80s...arnd 5F abv avg.
Transitional period for central FL as the short wave pinches off
into an closed low while pushing its way into the NE GOMex. Pattern
will evolve slowly as the jet pattern aloft is not particularly
strong...max H30-H20 winds in the 70-80KT range. Moisture will
increase thru the column as winds veer to the south, though model
soundings indicate it will do so in a "top-down" manner as the
departing ridge maintains a stubborn H85-H70 subsidence inversion
Full modification of the mid-lvl dry air not expected before
daybreak as H85-H50 dewpoint depressions overhead are on the order
of 30C. Even so, PWat values increasing to arnd 1.25" over most of
the CWA...as high as 1.25"-1.50" over the Treasure Coast/Lake-O
region by daybreak does warrant small PoPs in the fcst. Weak ripples
in the H85-H50 vort field will combine with increasing low lvl
frictional convergence as the H100-H70 mean flow becomes parallel to
the shoreline. As such, precip chances will be limited to the
coastal counties, and only 20pct chance at that. Srly flow and mcldy
skies will keep min temps in the L/M60s, about 10F abv avg.
Models continue in quite good agreement showing anomalous cutoff
low dropping southeast over the eastern Gulf of Mexico Wed and
bottoming out over the Florida Straits Thu before moving slowly
eastward. A line should move across the area Wed afternoon-evening
just ahead of an approaching occluded frontal boundary. The
strongest lift is indicated across the south half of the
peninsula, where the greatest chance for strong storms and
beneficial rainfall (1-2 inches) should be.
The parent 500mb low will carry a surface reflection with it then
secondary low formation is indicated Wed night off the southeast
Coast and then wrapping cyclonically back towards our southern
counties on Thu. This will keep the chance for precip going
overnight and Thu, along with a slight chance for lightning
MOS POPs continue near 80 percent Wed and Wed night, with likely
values even into Thu. Haven`t changed the previous forecast much
since there is little overall change in the models. Did nudge
coastal POPs to likely on Thu, following MOS and blended POPs.
Upstream mid level trough will kick out the deforming low rapidly
NE from the Bahamas up through the western Atlantic Fri-Sat. It
looks mainly dry on Fri in the wake of the departing low, then a
fast-moving cool front is shown sweeping through the area Sat
night. The GFS generates light precip amounts ahead of the front
and has POPs around 20 percent, but the ECMWF and blended POPs are
dry, so won`t carry any low threshold rain chances Sat.
Temps will be above normal Fri-Sat. The latest GFS and ECMWF MOS
guidance have trended cooler for Sun so have followed suit and we
should eke out one day with temps near or slightly below normal.
Temps will gradually moderate to above normal early next week as
high pressure shifts offshore and winds veer around to the east
Sfc Winds: Thru 21/13Z...E/SE AOB 3KTS. Btwn 21/13Z-21/16Z...bcmg
E/SE 8-12KTS. Btwn 21/23Z-22/02Z...bcmg SE 4-7KTS. Btwn 22/04-
22/06Z...interior sites bcmg SE AOB 3KTS.
Vsbys/WX: Thru 21/14Z...areas MVFR/lcl LIFR vsbys in BR/FG. Aft
22/06Z...slgt chc MVFR shras.
Cigs: Btwn 21/14Z-21/24Z...SCT-BKN FL040-060 BKN AOA FL120. Aft
Today-Tonight...A hi pres ridge spanning the ern seaboard will drift
east into the W Atlc as a broad storm system dvlps over the lwr MS
Valley/NW GOMex. The ridge axis will have the primary impact on the
lcl Atlc waters...generating a light to gentle breeze thru mid aftn.
However, as the storm system pushes into the Deep South/NE GOMex, it
will interact with the wrn flank of the ridge and tighten the lcl
pgrad. Winds increasing to a gentle to moderate SE breeze by late
aftn, then moderate to fresh in the predawn hrs. Seas 2-3FT thru
late evng...building to 3-4FT by midnight. Slgt chc of shras near
the coast aft midnight.
Wed-Sat...An approaching low pressure system will tighten the
pressure gradient Wed with southeast winds freshening to around 20
knots by evening and a small craft advisory will be necessary.
Additionally, showers and storms will become numerous. These
adverse boating conditions will continue Wed night and even into
Thu as the surface low tracks slowly across south Florida. Some
diminishing of winds is possible in the south on Thu near the low
center. Gradually improving conditions are forecast on Fri as the
low weakens. An offshore, west/northwest flow, is indicated Sat
before a cold front sweeps through Sat night and brings a brief
period of hazardous boating conditions.
.PRELIMINARY POINT TEMPS/POPS...
DAB 80 64 76 65 / 0 20 80 80
MCO 82 63 76 63 / 0 10 80 70
MLB 80 65 76 65 / 0 20 80 80
VRB 79 66 76 64 / 0 20 80 80
LEE 83 63 76 63 / 0 10 80 70
SFB 82 63 76 63 / 0 10 80 70
ORL 82 63 76 63 / 0 10 80 70
FPR 79 65 77 65 / 0 20 80 80