Area Forecast Discussion
Issued by NWS Melbourne, FL

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FXUS62 KMLB 200911
AFDMLB

Area Forecast Discussion
National Weather Service Melbourne FL
511 AM EDT Wed Sep 20 2017

.DISCUSSION...

Today-Tonight...
RAP40 analysis shows the band of low/mid moisture that wrapped
around the SW flank of Hurricane Jose has stalled in the vcnty of
Lake-O and extends east just north of the nrn Bahamas. The dry air
that was present over the I-4 Corridor on Tue has modified
somewhat, but a discernible moisture boundary still exists over
central FL: H100-H70 mean RH values btwn 60-70pct along the I-4,
increasing to 80-90pct over the Treasure Coast/Lake-O region. Evng
RAOBs map the moisture well with PWat values arnd 1.50" over
north/central FL... increasing to 1.75-2.00" over south FL.
KXMR/KTBW RAOBs also measured a noteworthy slug of dry air in the
H100-H85 lyr with avg dewpoint depressions running btwn 5-7C.
Nocturnal Gulf Stream shras reflecting the dry air as well with
limited activity has off the Treasure coast and none north of
Sebastian Inlet.

The H100-H70 wind pattern reveals a classic col just north of the
Bahama Bank, formed by a weak anticyclone extending from the GA
coast to the mouth of the MS river, a weak inverted trof extending
from the nrn Bahamas to the Yucatan Channel, a trof extending from
Tropical Storm Jose down the southeastern seaboard, and the wrn
flank of the Atlc ridge axis just east of the nrn Bahamas. The
resulting weak pgrad has allowed the mean lyr E/NE flow to diminish
to arnd 5KTS. Mid lvl flow not any stronger as an H85-H50 ridge axis
over central FL has collapsed the lcl pgrad.

The weak wind field will provide little in the way of dynamic
forcing for precip, not there is all that much in the way of
moisture to begin with.  The ridge axis over the nrn GOMex will
drift N as Tropical Storm Jose continues to creep slowly into the
Atlc waters off New England Coast. This will maintain a light E/NE
low/mid lvl flow overhead that will be roughly parallel to the
moisture band, so no sig change in moisture advection currently in
place.

A sharp subsidence inversion arnd H70 will cap vertical motion blo
15KFT, while the weak sfc/low lvl pgrad and the dry air north of the
Treasure Coast will promote the early formation of the east coast
sea breeze. No meaningful vort maxes overhead, and upr lvl
divergence is weak at best. Will keep slgt chc of shras in for the
from Osceola/S Brevard southward to give a nod to the moisture band,
but no mention of tsras. Max temps near climo...maxes in the
U80s/L90s interior, M80s along the coast. Onshore flow will keep min
temps a few degs abv climo...L/M70s.

Thu...Weak moisture convergence zone left over from old frontal
trough will keep PoPs elevated over our southern counties
(Treasure coast/Okeechobee). So will continue with PoPs there of
50 percent Thu and there will be a threat for locally heavy rain
near the coast. Much drier air across the north (I 4 corridor)
will produce a tight PoP gradient and will keep rain chances out
of the forecast north of Orlando. Max temps will reach the lower
90s over the north interior where more heating will occur with
upper 80s elsewhere.

Fri-Sat...There is little change to the forecast track of Major
Hurricane Maria which is expected to turn north Fri into a
weakness in the subtropical ridge carved out by "Jose". The
current NHC forecast keeps "Maria" well east of FL by about 500
miles this weekend. But another round of large swells impacting
the coast can be expected this weekend with high surf, a high risk
of rip currents and additional beach erosion. A High Surf Advisory
will likely be issued.

Easterly flow should increase some Fri and the moisture across the
south should spread northward and provide scattered PoPs areawide
Fri into Sat with a threat for locally heavy rainfall once again
esp near the coast. Temps aloft cool a couple degrees so there
should be a better chance for thunder. Winds becoming northeast
and breezy by Sat especially along the coast with sustained winds
near 20 mph. Max temps in the M80s along the coast, upper 80s/near
90F over the interior. Min temps L/M70s.

Sun-Tue...As "Maria" lifts north of our latitude Sunday, northerly
flow will advect a drier airmass down into the area so rain
chances will decrease to 20 percent or less through early next
week. Have also removed any thunder. The pressure gradient will
weaken considerably as "Maria" pulls away so winds will become
quite light Mon-Tue. High temps will be near climo in the upper
80s/near 90.


&&

.AVIATION...Thru 21/12Z.
Sfc Winds: Thru 20/14Z...E/NE 3-5KTS. Btwn 20/15Z-20/18Z...bcmg E/NE
8-12KTS. Btwn 20/01Z-20/04Z...bcmg E/NE 4-6KTS.

Vsbys/WX/Cigs: Thru 20/12Z...E of KMLB-KOBE slgt chc MVFR shras.
Btwn 20/18Z-20/24Z...S of KTIX-KSEF chc MVFR shras/slgt chc IFR
tsras. Aft 21/00Z...E of KMLB-KOBE chc MVFR shras.

&&

.MARINE...
Today-Tonight...Long pd swell generated by Hurricane Jose continues
to slowly diminish, now arnd 5FT down from 7-8FT 24hrs ago as
measured by buoy009...nearshore Scripps buoys measuring swells btwn
3-4FT. A weak high pres ridge over the NE GOMex will combine with a
weak inverted trof over the nrn Bahamas to generate a light to
gentle E/NE breeze over the lcl Atlc that will enhance the swell by
about a foot. Combined seas 4-6FT nearshore and 5-7FT offshore thru
sunset, subsiding to 3-5FT and 4-6FT respectively overnight.
Offshore seas technically in the SCA range, but with sfc/bndry lyr
winds hovering around 10KTS or so, overall conditions no longer
warrant continuation of the SCA. Will keep a cautionary statement
out for the Gulf Stream waters.

Thu-Sun...High pressure ridge to the north of the waters will
produce an onshore flow around 10 knots Thu, except near 15 knots
near a weak trough/convergence zone lingering over the
southern/central waters.

Even though Hurricane Maria will track to the north well east of our
waters this weekend, the pressure gradient will start to increase
by Fri with east winds around 15 knots expected. Winds from the
northeast 15-20 knots are forecast over the weekend.

Swells from "Jose" will continue to gradually subside Thu then
swells from Hurricane Maria should start to build seas again by
Saturday. With winds increasing slightly over the weekend, Small
Craft Advisories are likely as seas build to 7-9 feet nearshore
and up to 15 feet offshore by Sunday.

&&

.HYDROLOGY...
Minor to moderate flooding along the Saint Johns River near Cocoa,
Geneva, Sanford, Deland and Astor are forecast to change little
into late week. The Saint Johns River above Lake Harney near Geneva
is forecast to remain in major flood stage.

&&

.PRELIMINARY POINT TEMPS/POPS...
DAB  87  72  88  75 /   0   0  10  20
MCO  90  74  91  74 /  10  10  10  20
MLB  86  74  88  76 /  20  20  30  30
VRB  86  73  88  75 /  20  20  50  30
LEE  92  72  93  74 /   0   0  10  10
SFB  90  72  91  74 /  10   0  10  20
ORL  90  73  92  74 /  10   0  10  20
FPR  86  73  88  75 /  20  20  50  30

&&

.MLB WATCHES/WARNINGS/ADVISORIES...
FL...None.
AM...None.

&&

$$

SHORT TERM...Bragaw
LONG TERM....Kelly
AVIATION...Bragaw



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