The Current Weather ‘Down Under’: Tropical Cyclone Ita’s Assault on Australia

April 11th: Sara Ganetis led a discussion titled “The Current Weather ‘Down Under’: Tropical Cyclone Ita’s Assault on Australia”
          The discussion began with an overview of the current weather over the Northeast U.S., mainly highlighting the well-received warmth experienced during the day prior to a cold frontal passage. Central Park reached 74F, for example. A topic that has been being addressed every week is the possible onset of El Nino based on the progression of a warm anomaly present near the thermocline slowly reaching the surface of the Eastern Pacific which can be seen by animating the temperature-depth plot from CPC. While it may seem apparent that El Nino is imminent, there is still the issue of the Spring predictability barrier that may mean that although the signal seems strong right now, there may be other atmospheric responses that aren’t in-line with El Nino.
The discussion then switched gears to discuss a few notable events of the past week especially the spreading of wildfires in the region. Research is still ongoing at SBU looking into the predictability of high fire threat days which is tied to atmospheric moisture (or lack-thereof) and gusty winds. Last Sunday evening, 6 April, there was a wildfire at a State Forest in southern NJ that was identified via radar and discussed through social media outlets by local NWS WFO’s (Mount Holly & NYC). What was interesting about this particular fire is that as the sun set, a strong temperature inversion was present that persisted overnight. That meant that the smoke and particulate matter could not be vertically dispersed but instead had to travel horizontally which it did with a strong southerly wind northward to NYC and points west. Smoke was able to be smelled far away from where the fire originated and an air quality alerts were issued for several counties. Some snapshots can be found here.
Tropical Cyclone Ita formed northeast of Australia in the South Pacific in waters with SSTs exceeding 30C and favorable environmental conditions (i.e. relatively moist air mass and far-removed from the reaches of extratropical waves that could enhance the environmental shear). At the time of the discussion, it had already crossed the Great Barrier Reef and made landfall (~10 UTC 11 April) in between the cities of Cooktown and Cairns on the eastern side of the Cape York Peninsula in Queensland, Australia with estimated wind speeds of 125 kts. A loop of the IR imagery shows that it developed a very small, symmetrical eye before making landfall and having that eye fill-in a bit due to frictional convergence, steep terrain and removal from surface fluxes. An ASCAT pass prior to landfall (2002 10 April) showed the fastest winds near the eye as well as along the edge of the continent in what looked like a barrier jet. The Cooktown station time series of pressure, sustained wind speed and wind gusts shows that the pressure decreased to around 983 mb around 15 UTC 11 April and the sustained wind speed (gusts) got to >40 (65) kts. The JTWC forecast track of TC Ita had the system recurving back out to sea in the South Pacific. A look at the GFS deterministic forecast showed that while Ita had persisted for so long and moved due west under the influence of a broad area of high pressure over the Coral Sea, a trough was moving eastward and expected to shift the track more southward and then eastward with time. A 250-hPa jet also developed poleward of Ita suggesting that it will have undergone extra-tropical transition. Ita was an interesting tropical system that produced some interesting discussion.
The weather discussion concluded with a local forecast for that Friday (11 April) evening and then the system forming over the weekend and expected to drag its cold front across the region Tuesday-Wednesday. The SBU-WRF was used to discuss the near-term forecast for that evening, mainly how much precipitation should be expected behind the cold front? The answer was different for both the NAM-WRF and GFS-WRF, with the NAM-WRF being the drier solution. Comparing the current regional radar with that of the NAM-WRF and GFS-WRF forecast reflectivity showed that the GFS had much larger areas of higher reflectivity than the NAM and the NAM was closer to reality so perhaps we wouldn’t expect much rain with the cold front. The next system was expected to form lee of the Rockies in a developing trough out ahead of a cold, Canadian air mass and likely cause some severe weather in the Plains around Sunday. The fact that the southerly flow out ahead of this system is forecast to be strong and persistent and a high pressure off the East Coast should allow for an influx of moisture towards the coast means that there should be some anomalously high precipitable water values and thus very heavy rains Tuesday-Wednesday. We quickly compared the deterministic GFS with the ECMWF QPF forecasts to find that the ECMWF was slower and weaker with the precipitation totals than the GFS. That will have to be something we keep in mind as we get closer to the event. Check out the weather discussion page for a few visuals to go along with the above text.