Rain and thunderstorms from the system could arrive as soon as overnight Tuesday. The potential for flooding and, if ingredients come together, tornadoes will increase on Wednesday. The heaviest rain and highest chance for severe weather may occur Wednesday afternoon and night, although this timing is subject to change.
A flash flood watch has been issued for the region between Wednesday and Thursday morning for two to four inches of rain and locally higher amounts. The risk of flooding is intensified due to the amount of rain the region has already had this month. Over nine inches have fallen in Washington, more than twice the average.
The track forecast and how the storm remnants will evolve
The center of Ida’s remnants could pass through D.C., or up to 50 miles north, or 50 miles south; don’t focus too much on the black line in the above tracking map. The track will be refined with time. But it’s a good bet that our region will be dealing with some level of significant impacts, given the proximity of the storm’s center. These will begin by as early as overnight Tuesday, especially in our western areas.
As Ida’s remnants work northeast along the spine of the Appalachians, they will approach and merge with a cold front dropping down from the Mason-Dixon Line by Wednesday morning. At this stage, Ida’s post-tropical core will transform into a hybrid type of storm system, a process called extratropical transition. The hybrid storm will take on the structure of an extratropical cyclone, complete with a warm front and cold front. The early phase of this transition is shown in the graphic below.
Ida’s remnants will bring lots of spin energy in the upper atmosphere and low pressure aloft. On Wednesday night, Ida’s upper-level low and a dip in the jet stream over the Great Lakes will begin to merge. The combined, larger area of low pressure will enhance the widespread uplift of air over the Mid-Atlantic, and also generate a pocket of stronger winds aloft, creating wind shear that could intensify and sustain thunderstorms. This interaction is shown in the graphic below.
Additionally, an embedded region of fast flow in the jet stream — called a jet streak — will extend from New England into the northern Mid-Atlantic. The southwest quadrant of this feature (shown below) is a favored location for more intense upward motion in the atmosphere, potentially intensifying rainfall.
The hazards from Ida’s remnants break down into three categories: heavy rain, strong wind gusts and tornadoes. Let’s take each of these one-by-one.
Heavy rain: This is the most widespread anticipated hazard, given very humid air imported by ex-Ida’s tropical air mass, and increased moisture pulled northward into the warm sector of the transforming storm.
The flash flood potential for Washington is considered a level 3 out of 4 risk by the National Weather Service, as shown below. The locus of heaviest rain accumulation may in fact lie just to the north and west of the District, as moist low-level winds from the east-southeast ascend high terrain and rainfall production gets a boost, generating what’s known as orographically enhanced rainfall.
The graphic below shows predicted, general amounts (based on forecast model consensus) in the two- to four-inch range, over a period of 24 to 36 hours, across the Washington region. Note, however, that a slight southward shift in the center of former-Ida will bring our region closer to four to six inches.
Rainfall will fall in the form of several heavy showers, as thunderstorm cells organize into bandlike structures in the warm sector of the storm. Localized spot totals of over six inches cannot be ruled out as multiple bands traverse the same regions. More extreme totals of six to 10 inches are most likely in the mountainous regions of the West Virginia panhandle, far western Maryland and extreme northwestern Virginia.
Gusty winds: Much of Ida’s wind energy will be spent by the time the core passes through or nearby the D.C. region, but general gusts from the south and southwest will peak in the 25 to 30 mph range during the evening and overnight hours. Higher gusts to 40-plus mph are possible in thunderstorms embedded within bands of heavy rain. Stronger wind gusts will prevail in the higher terrain north and west of the District.
Once Ida’s core passes to our northeast, a cold front on the backside of the storm may cause a brief resurgence of wind gusts early on Thursday morning.
Tornadoes: Tropical remnants, after moving inland, are notorious for spinning up tornadoes, particularly in the eastern quadrant. At this point, the Weather Service’s Storm Prediction Center has placed our region in a level 1 out of 5 risk Wednesday, covering low-end probability of damaging wind gusts and tornadoes.
As Ida morphs into an extratropical cyclone, more unstable and humid air is expected to surge over the Mid-Atlantic, enabling the development of heavy showers and thunderstorms. The increase in wind shear and local spin generated along a warm front may conspire to up the odds of rotating storm cells, and thus tornado formation.
The greatest threat for tornadoes is later in the day on Wednesday. Tornadoes from inland tropical systems tend to be small, short-lived and weak, and also notoriously difficult to predict.
It’s possible that the rainfall threat and severe storm threat may be upgraded to higher risk levels in the next 24 to 36 hours. In addition to the flash flood watch in effect, it’s also possible that the region will be placed under a Tornado Watch.
Capital Weather Gang will update the weather threats to our region over the next two days as Ida’s track and interaction with preexisting weather systems becomes more clearly focused.
Jason Samenow contributed to this report.