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.
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.
Now move forward 24 hours to the forecasted upper level chart for tonight at the same time (00z 9/19 or 8pm EST).
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.
Extending out to 48 hours out, the system moves northward into Canada.
Analyzing the lower levels associated with this timeframe:
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.
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!