Quantitative Precipitation Forecast
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FOUS30 KWBC 190056

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Sun Apr 18 2021

Day 1
Valid 01Z Mon Apr 19 2021 - 12Z Mon Apr 19 2021


Favorable mid/upper level jet dynamics will support broad large
scale forcing for ascent across portions of northern Florida
peninsula near a surface front sagging slowly southward. Gulf
moisture lifting northward into the low level boundary will be
characterized by precipitable water values ~1.75 inches with
anywhere between 25-35 kts of low-level inflow. Normally the
spring dry season, the moisture parameters are rather anomalous
for the latter half of April -- including PW and 850 mb moisture
flux (around 2 and 2-3 standard deviations above normal
respectively). With sufficient instability, moisture, and inflow,
a couple rounds of moderate to heavy rainfall will be possible
across the northern and central Florida peninsula between now and
Monday morning.  An additional 2-4+ inches is possible. This
rainfall is likely to fall on areas that have seen much above
normal precipitation over the last 2 weeks, as the 14-day
anomalies are between 300-400 percent over many locations in North
and Central Florida.  Multi-radar, multi-sensor rainfall estimates
between Ocala and Palm Coast show spots of 6-7" over the past 48
hours through 8 pm EDT Sunday.  Given the mesoanalysis, radar
reflectivity, and guidance trends, a slight southward adjustment
was made to continuity.


Day 2
Valid 12Z Mon Apr 19 2021 - 12Z Tue Apr 20 2021


A stalled cold front draped across the FL Peninsula will serve as
a focus for rounds of showers and thunderstorms with heavy
rainfall Monday and Monday night. As this front crawls slowly
southeast across the state, ascent through low-level convergence
will be aided by persistent upper level diffluence as a jet streak
remains anchored to the north, and weak PVA/height falls as
periodic shortwaves traverse the flow from west to east. Broad
cyclonic flow across much of the eastern CONUS will maintain
unidirectional westerly flow across the Gulf of Mexico and into
FL, advecting onshore PWs in excess of 1.75", above the 90th
percentile for the date according to the SPC sounding climatology.
Persistent warm and moist advection through D2 will also push warm
cloud depths towards 4000m as MUCape above 1000 J/kg as the
sounding becomes deeply saturated. While individual storm motions
may reach 30 kts to the northeast using 0-6km mean wind as proxy,
training of echoes is likely on the unidirectional, and boundary
parallel, flow. Additionally, regeneration of cells over the Gulf
of Mexico is likely where instability maximizes, and this will
enhance the rainfall potential across the Peninsula.

Recent rainfall across parts of the Peninsula has been more than
300% of normal the last 7-day, and parts of the area have 0-40cm
soil moisture now exceeding the 98th percentile. However, there
continues to be uncertainty into exactly where the heaviest rain
may align, and how often rain rates, which could exceed 1"/hr
according to the HREF probabilities, will occur. For these reasons
the MRGL risk was left unchanged from the previous forecast, with
just a subtle placement adjustment to account for model trends.


Day 3
Valid 12Z Tue Apr 20 2021 - 12Z Wed Apr 21 2021


Rinse and repeat seems to be the theme for D3 as a slow moving
front, upper jet level diffluence, and weak shortwave impulses
embedded within the mid-level flow combine to produce heavy
rainfall across Florida. Although the front will try to progress
southeastward D3, there is some indication that increasing
instability Tuesday will stall the front and may even cause it to
waver back to the north, and the MRGL risk has been adjusted to
account for this potential. PWs still surging from the west at
1.75", or above the 90th percentile for mid-April, will maintain a
very moist column with 4000m of warm cloud depth and MUCape to
1000 J/kg or higher. With persistent westerly flow parallel to the
front, training of showers and thunderstorms with rain rates in
excess of 1"/hr are again possible across some of the same areas
that will receive heavy rainfall on Monday.

The main difference Tuesday which may enhance rainfall is a subtle
tightening of the mid-level heights as the main longwave trough
amplifies to the northwest in response to a neutral tilting
shortwave over the Great Lakes. This will drive some enhanced
mid-level flow and stronger moist advection, in conjunction with
more robust RRQ diffluence as the tail of the jet streak shifts
eastward. Guidance indicates a mid-level omega maximum developing
Tuesday, and both ECENS/GEFS probabilities and WSE plumes suggest
heavy rainfall across much of central FL adding up to more than 3
inches in some areas. Considered raising a SLGT for a narrow
corridor of the central Florida Peninsula, but after coordination
with TBW and MLB, opted to hold on to the MRGL at least one more
cycle due to uncertainty in rainfall expected on Monday (D2) which
will impact the soil moisture and flash flood potential on Tuesday.


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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