The making of a perfect rain storm over the East Coast and Applachians
This week and especially today, all eyes are on the East Coast where two systems are converging for a massive deluge. The widespread rain is the textbook perfect rainstorm for this portion of the US and where developed systems can be examined the best in their interactions with different air masses. In this blog I will look at the current state of the system, 24 hours prior, and in the future (using the NAM 00z run on 9/18). I will give a quick look at a paper and how this system conformed exactly to scientific method which helped lead to its predictability.
I will start with an upper level map valid at 00z on 9/18 (8pm EST) last night.
Notice a few things on this 300mb upper level chart: a ridge in place over the western Atlantic, a amplifying trough over the central US with multiple shortwaves, and a cut off wave of energy (weak shortwave) highlighted in the Gulf.
Lets now drop down further towards the surface with an 850mb height map that also has precipitable water plotted on it. The higher the colors, the more moisture in the atmosphere, and therefore, the more rain the atmosphere over those locations is capable of dropping. This is for the same time as the above map.
The main low, now weakened, over Quebec is associated with the main shortwave/jetstreak we saw over the Great Lakes. The cold front associated with that low extends further SW into the Mississippi Valley where it has just caught another weak low. This low was associated with the weak wave of energy that was circled in the previous map. Notice how the warm sector (the region between the warm and cold fronts) contains much more moisture than the other areas and is streaming more Gulf moisture northward.
Now move forward 24 hours to the forecasted upper level chart for tonight at the same time (00z 9/19 or 8pm EST).
Take notice how the wave of energy that was in the Gulf is now part of the main amplified trough over the Great Lakes. The energy has been absorbed by the main system and enhanced the surface features which you can see below. Also notice how the ridge over the western Atlantic has remained in place and not budging.
The 850mb chart also shows the secondary low over the US strengthening and moving more east/north along the cold front and where the best upper level divergence/surface convergence exists.
Keep in mind, this is a forecast map for 8pm EST tonight. Notice how the moisture streams much farther northward now and is much taller and skinnier than before. This confines the heaviest rainfall to a corridor parallel to the East Coast.
With a strong high over the western Atlantic, combined with a strong low over the eastern US as such, parcel trajectories (in the red arrows) all point and converge over the East Coast. This is shown by higher precipitable water indices and coinciding heavy precipitation.
Extending out to 48 hours out, the system moves northward into Canada.
The energy associated with the shortwave we are interested in has rotated around the upper level trough and pushed the low into Canada. Another shortwave has since moved into the Mississippi Valley but is much weaker and has no energy to work with. The ridge in the eastern Atlantic has prevented the entire trough from pushing eastward and keeps it in nearly the same spot.
Analyzing the lower levels associated with this timeframe:
As the low moves NE into Canada, the cold front extends offshore now and has taken its rain with it (minus Florida and extreme NE Maine). The paths the two lows took are highlighted with the arrows (brown the secondary low, purple the original low).
Keeping in mind that the first low (the purple track) crossed the area ahead of the secondary low, compare it to the graphic from Bjerknes 1922 model of the secondary low formation. It followed very nicely and is considered to be textbook.
From Bjerknes and Solberg 1922. The secondary low develops from the original low and then becomes separate, taking a different track.
These secondary lows can form many different ways including the above pictured warm front being over ran by the cold front away from the main circulation (not occluding), a stronger shortwave following a lead shortwave that weakens, or the example we just described, a broken off piece of energy/shortwave being over ran by a stronger shortwave. Being able to pick up these details from upper level forecast maps can help depict heavy rain events and give you a step up on the competition. Hopefully this discussion helped your upper level chart skills and relating them to what is occurring on the ground!
Chip
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