Sunday, September 9, 2018

Hurricane Florence Threat Coming Into Focus: Life-Threatening Southeastern U.S. Impacts of Storm Surge and Catastrophic Flooding; Sunday Regional Record Daily Rainfall, Precip Summary

GOES-16 visible satellite loop of Hurricane Florence on September 9, 2018


Below is reposted in full the 11 p.m. AST (11:00 pm EDT) National Hurricane Center (NCH) discussion on Hurricane Florence, along with some interspersed images and captions (with appropriate links).

Bottom line: Hurricane Florence is in the process of undergoing a rapid intensification that is forecasted to bring it back to a category 4 storm by Tuesday. The models are tightly clustered on a landfall around that old hurricane magnet of Cape Fear, N.C.

NWS NHC five-day track / cone of uncertainty for Hurricane Florence along with expected strengthening to a major hurricane ("M"); Advisory 43, 11 p.m. AST September 9, 2018.


However, at that point, the tropical cyclone will slide under a large ridge and stall -- somewhere between eastern North Carolina and western Virginia / West Virginia -- and rain itself out, dropping FEET of rain (10 to 20 to 30+ inches) (akin to Hurricane Harvey, even if the synoptic reason is different) and causing catastrophic flooding.

The European and GFS 12Z 9/9/2018 precipitation totals for an identical portion of the eastern United States between hours 84 and 204.


Impacts here in the D.C. area in the Friday - Monday time frame are uncertain -- possibly flooding rains or possibly very little. As it is, we've been soaked with heavy rainfall lately.

Hurricane Florence as seen on Sept. 5, 2018 during its first period as a strong hurricane.


Hurricane Florence Discussion Number 43
NWS National Hurricane Center Miami FL AL062018
1100 PM AST Sun Sep 09 2018

Florence's satellite appearance has continued to improve quite markedly since the previous advisory. An eye was evident in GOES-16 high-resolution infrared imagery and other channels between 2300-0000 UTC, but it became cloud covered immediately thereafter due to a strong burst of deep convection in the southern and eastern eyewall where cloud tops colder than -80C and an abundance of lightning activity was observed.

Since that time, the CDO has expanded and become more circular, outflow has increased and become more symmetrical, and an eye has begun to re-appear. The initial intensity has been increased to 80 kt for this advisory based on a subjective Dvorak intensity estimate of T4.5/77 kt from SAB and NHC objective intensity estimates ranging from 77 kt to 87 kt.

The initial motion estimate is 280/06 kt. Once again, there is no significant change to the previous forecast track or synoptic reasoning. The models appear to be getting better dialed in on both the location and strength of the developing blocking ridge in the vicinity of Bermuda during the next 4 days as the mid-latitude flow amplifies across the CONUS and the northwestern Atlantic.

In fact, the global models are now in very good agreement on forecasting Florence's upper-level mass outflow being deposited to the north and east of the hurricane, which will act to further strengthen the blocking ridge and help to drive the hurricane northwestward toward the southeastern U.S. coastal region. The new NHC model guidance is even more tightly packed about the previous forecast track, with less than a 90 n mi cross-track spread at 72 h and less than 120 n mi spread at 96 h, just prior to expected landfall.

Therefore, the new official advisory track is essentially just an update and extension of the previous forecast, and lies between the nearly juxtaposed HCCA and FSSE corrected consensus track models.

Now that Florence has developed an inner-core ring of deep convection, which has insulated the eye from intrusions of dry air, rapid intensification appears likely to begin soon and continue for the next 36 hours or so due to the expected very low vertical wind shear conditions, dual outflow jet pattern that will be developing, and very warm SSTs of 29-29.5 deg C beneath the hurricane.

The most favorable combination of the aforementioned factors will occur in about 48 h, and that's when Florence is likely to achieve its maximum intensity. After 72 hours, the wind shear is expected to increase to around 10-15 kt from the south or southwest, and the dual outflow pattern is forecast to change to only a single poleward outflow pattern.

This slight degradation in the upper-level environment, along with slightly cooler SSTs, is expected to result in a gradual weakening of the powerful cyclone. However, Florence is still forecast to be near category-4 strength when the dangerous hurricane makes landfall. The official intensity forecast is an average of the intensity forecast from the corrected consensus models HCCA and FSSE, with the latter explicitly forecasting a peak intensity of 134 kt in 72 hours. It is also worth noting that the model guidance is also significantly increasing the size of

Florence's wind field over the next few days, and the official forecast reflects this trend.

The NOAA G-IV jet will conduct another synoptic surveillance mission early Monday morning in support of the 1200 UTC model cycle, and these flights will continue through Tuesday. In addition, upper-air stations across portions of the central and eastern U.S. are conducting special 0600 UTC and 1800 UTC radiosonde launches to collect extra data for the numerical models. Hopefully these data will help improve the track and intensity forecasts.

Key Messages:

1. There is an increasing risk of two life-threatening impacts from Florence: storm surge at the coast and freshwater flooding from a prolonged heavy rainfall event inland. While it is too soon to determine the exact timing, location, and magnitude of these impacts, interests at the coast and inland from South Carolina into the mid-Atlantic region should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials.

2. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week. These swells will result in life-threatening surf and rip currents.


INIT 10/0300Z 24.6N 57.7W
80 KT 90 MPH
12H 10/1200Z 24.9N 59.1W
90 KT 105 MPH
24H 11/0000Z 25.6N 61.5W
105 KT 120 MPH
36H 11/1200Z 26.5N 64.4W
120 KT 140 MPH
48H 12/0000Z 27.8N 67.6W
130 KT 150 MPH
72H 13/0000Z 31.2N 73.6W
130 KT 150 MPH
96H 14/0000Z 34.0N 77.5W
120 KT 140 MPH
120H 15/0000Z 35.2N 79.0W

Forecaster Stewart


As for today -- Sept 9, 2018 -- it rained heavily around the Mid-Atlantic including here in the D.C. and Baltimore areas as a quasi-stationary front and waves of low pressure were held in place by a blocking high over Quebec. All three regional airport climate stations had daily record rainfalls. 

Below are some radar imagery and weather advisories from earlier today:

Sterling (LWX) NWS radar in standard base mode reflectivity looped 6:50 a.m. - 7:25 a.m. EDT Sept 9, 2018


LWS NWS radar in standard composite mode looped 6:45 a.m. to 7:20 a.m. EDT Sept 9, 2018


NECONUS composite radar mosaic looped 0928 - 1038 UTC Sept 9, 2018


Sterling (LWX) county warning area (CWA) weather advisories (with legend) 11:07 EDT Sept 9, 2018


LWX CWA weather advisories (with legend) updated 2:10 p.m. EDT Sept 9, 2018


Here are the precipitation totals for the day and year-to-date:

Day: 1.59" (daily record) (old record: 1.48" in 1880 -- pre-airport record)
Month- (and season-) to-date: 5.74" +4.74" (1.00")
Year-to-date: 45.84" +18.44" (27.40")

Day: 2.23" (daily record) (old record: 1.35" in 1950)
Month- (and season-) to-date: 3.08" +1.99" (1.09")
Year-to-date: 48.34" +19.40" (28.94")

Day: 1.40" (daily record) (old record: 1.34" in 1999)
Month- (and season-) to-date: 2.38" +1.31" (1.07")
Year-to-date: 45.64" +16.57" (29.07")

Day: 1.90"
Month- (and season-) to-date: 3.01" +1.86" (1.15")
Year-to-date: 45.25" +16.73" (28.52")

Note: KDMH records only go back to April 30, 1998; does not yet have a full 30-year normal base period.

We are way above normal for year-to-date. Just for reference, the wettest full calendar year ever in D.C. (KDCA) is in a pre-airport period record of 61.33" in 1889.

The second wettest is much more recently at 60.83" in 2003.

What's more, the only other 60-inch plus year was 60.09" in 1878. Those are 19th Century / pre-modern period records.

For Baltimore (KBWI), the wettest full calendar year ever was 62.66" in 2003 and the second wettest was 62.35" in 1889, a pre-airport record.

Of note, in 1887 -- when D.C. recorded its 3rd ettest ever at 60.09" -- Baltimore had 50.19". (The big differences were in July and August: 8.37" and 8.89" vs 4.76" and 4.82", respectively.)

Full records for both locations go back to January 1871. Airport records began in Aug 1941 (or Aug 1945) for D.C. and Jan 1950 for Baltimore.

For Dulles Airport (KIAD) -- where records only go back to April 1960 and continuously only since January 1964 -- the wettest year by far was 65.69" in 2003. The second wettest is 59.05" in 1972 and third wettest was 58.09" in 1996.

By the way, the highs today were as follows:


Just three days ago, temps were in the 92F to 95F range for highs.

OK, I'm going to end this entry now. My next planned update will be late Monday or Tuesday.


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