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Isaias Observations


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I didn't know my weather console came with jokes! 

That map destroys any snowfall total map we had this past winter. How sad is that?😆 

Welp, we got slammed today in upper Darby. Was a little quiet when I got in, was able to post some things, and then the kettle boiled over. Notions flood prone spots, well they flooded.... numerous wa

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1.34" so far in East Nantmeal....heaviest rain so far has remained to my east...0.56" in last hour. The numbers I will report today are from the Vantage Pro which normally under reports rain. I will update tomorrow when I am back in Chesco and check the manual gauge. I am riding this out down in Sea Isle....winds are howling....unfortunately my vantage vue is having battery problems so hoping we get enough light to get it back on line to measure some of these gusts!

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The severe weather threat for Tornado Watch 416 continues.

   SUMMARY...The tornado threat is expected to increase into southern
   New Jersey, northern Delaware, and far eastern Pennsylvania over the
   next 1-3 hours.

   DISCUSSION...An impressive, long-lived arc of supercells just
   north/east of the tropical cyclone track continues to rapidly shift
   north. Supercells with sporadic couplets/tornadoes currently stretch
   from eastern MD/northern DE southeastward, with cells now
   approaching coastal southern NJ.

   The DIX VWP has shown a substantial increase in low-level shear over
   the past couple hours, with 0-1 km SRH quickly approaching 300
   m2/s2. Shear values will likely max-out just before the arcing line
   of storms arrives later this morning. 

   12Z soundings also show impressive low-level CAPE, primarily
   concentrated below 500 mb. Although warming aloft will occur with
   the approach of the tropical cyclone, areas that experience mid-70 F
   dewpoints will maintain a tornado risk.

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We have upgraded to a HIGH risk from roughly the DC to Baltimore 
corridor into portions of eastern PA. Potentially life threatening 
flash flooding is possible across this corridor this morning into 
the afternoon hours. Despite the quick forward speed of Isaias the 
interaction with the approaching trough and upper jet streak will 
allow for significant rainfall to occur in a relatively short time 
period. The last several runs of both the HRRRv3 and HRRRv4 are 
indicating a swath of 3-6" of rain, with localized 8" totals 
stretching from near DC to Baltimore and then up into portions of 
eastern PA. Both of these models seem to have a pretty good handle 
on the current structure of the rain bands when comparing the 
forecast reflectivity to the observed radar data. Both of these 
models also indicate hourly rainfall magnitudes getting into the 
1-2" (locally 3") range for a few hours...so this will not be a 
drawn out 3-6", but rather a quick punch of heavy rainfall. 

Given the limited instability would typically question if rates 
would get this high. But in this case the synoptic interaction of 
the system with the upper jet is intense enough that we very well 
may be able to realize these higher rates, even without a truly 
unstable environment. Given this, and the fact that it is hard to 
ignore the impressive signal seen on the last several HRRRv3 and 
HRRRv4 runs...think the upgrade to the High risk is warranted. 
There is some east/west spread in exactly where this heaviest 
rainfall axis will end up...but confidence is pretty good that it 
will occur somewhere withIN the relatively narrow High risk that 
we have issued. 
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