Western Pacific Tropical Analysis: July 30, 2017

Three systems are currently active across the Tropical Western Pacific, although two are currently inland. As it has done for over a week now, the long standing Typhoon Noru continues to spin over the Western Pacific waters, now meandering near the transition waters between the tropics and subtropics. Former Typhoon Nesat is now an entry level tropical storm over eastern China after crossing Taiwan. Tropical Storm Haitang is following closely on the heels of Nesat and now located inland over Taiwan. Some additional brief development may occur in the subtropical waters well east of Noru courtesy of a reverse-oriented monsoon trough, but the bulk of this entry will be dedicated to Typhoon Noru.

***NOTE: While I would consider myself well-learned in meteorology, I am still a student with more to learn before becoming a degreed meteorologist. This forecast is not from an official source and should not be treated as such. For official information, please refer to your local weather agency.***

In a season thus far of short-lived tropical cyclones, Typhoon Noru has stood out as a significant aberration. Noru is quickly closing in on ten days as an active tropical storm. With the system now positioned by JMA at 22.9ºN, 141.4ºE as of 12Z July 30, Noru is now south of the Tropic of Cancer for the first time in its life. This southward drop into the transition area between the tropics and subtropics is also accompanied by warmer, more heat laden waters and a more moist surrounding airmass, something Noru has been lacking the past several days. Coupled with light upper level winds, it should be no surprise to see Noru strengthening.

With 12Z intensity estimates of 80 kt and 95 kt from JMA and JTWC, respectively, the system has rapidly intensified from just under typhoon strength to 5 kt below each agency’s T5.5 intensity. On first glance, this sudden burst of intensification appears somewhat surprising, but perhaps it shouldn’t be. 37 GHz microwave imagery from about 48 hours previous to this post revealed good structure featuring a cyan eyewall ring, despite Noru struggling with the dry surrounding airmass at the time. In the presence of favorable conditions, a cyan ring on 37 GHz microwave imagery is usually an indication that rapid intensification is about to commence. Subsequent Coriolis and AMSR2 passes over the next 24 hours showed the continued presence of this cyan ring. Over the past 12-18 hours, Noru has moved into a more favorable environment, as mentioned in the paragraph above, and it appears that Noru’s surprisingly good structure has allowed for rapid intensification. I can’t quite place why Noru’s structure was able to consolidate while struggling with dry air and weakening below typhoon intensity, but if I were to speculate, the light to non-existant wind shear present over the system allowed the structure to remain undisturbed, despite the repeated and almost constant dry air intrusions. With newfound favorable conditions, Noru has intensified to its strongest yet. Constrains obviously come into play, but raw and instantaneous Data Ts are currently up near 7.0.

Noru’s drop to the south is in direct response to a subtropical ridge strengthening just to the system’s north, shoving the system south. This now-established ridge will be the primary steering mechanism for Noru over the next couple of days, although Noru will not be moving much. A slow westward motion is the primary expected motion for this period. After this, a passing mid-latitude trough will begin eroding the steering subtropical ridge, and Noru’s steering will again break down. A slow drift with a poleward component appears to be the preferred solution appears to be a common theme amongst guidance, but the degree of lateral movement at the same time is uncertain. Such motion could eventually make a big difference for Noru’s ultimate future, and a further westward positioning could eventually result in a trough capture that sends Noru into Japan. European guidance has consistently favored the western drift over an eastern one, and most of the strongest members amongst ensemble suites lie near the western side of their spreads. Considering Noru’s ongoing rapid intensification, I am biasing my track philosophy towards the left side of the guidance envelope, although the weak steering and large spread inherently leads to a low confidence forecast. My track would be a little left of, but very near both JMA’s and JTWC’s track forecasts.

Intensity forecasting is again tricky. Noru is and will be over waters warm enough to support a rather intense storm, but with the system’s slow movement, upwelling could eventually become an issue. Peak intensity may actually come in the near term with the current bout of rapid intensification, which may bolster Noru near or even surpassing the threshold for super typhoon intensity. Inner core dynamics may come into play following this bout of rapid intensification however, and it wouldn’t surprise me to see eyewall replacement begin in about 24 hours or so. Upwelling may come into the picture at this point, but I do expect other factors to remain favorable for Noru. At this point I expect Noru to be above SSHWS category three for the few days.

Tropical Storm Haitang has recently made landfall over Taiwan. The system was not particularly strong at landfall, but the system does feature some very deep convection. The one-two punch of Nesat and Haitang for Taiwan and eastern China could result in numerous flooding issues.

Extending to the south and east of Typhoon Noru is a reverse-oriented monsoon trough. Guidance has backed off considerably regarding the quantity of systems consolidating and emerging from the feature, but it still appears likely that at least one system will manage to develop well to the east of Noru. Considering the elevated latitude that the reverse-oriented monsoon trough is currently found at, any emerging system will likely remain on the weaker side before shooting off into the mid-latitudes.

My next scheduled post is slated for Wednesday. Until the next entry is posted, analyses and updates in forecast philosophy will arrive in the comments section.

7 thoughts on “Western Pacific Tropical Analysis: July 30, 2017”

    1. Here is JTWC’s Prognostic Reasoning:

      WDPN31 PGTW 302100
      MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
      SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 07W (NORU) WARNING NR
      41//
      RMKS//
      1. FOR METEOROLOGISTS.
      2. 6 HOUR SUMMARY AND ANALYSIS.
      SUPER TYPHOON (STY) 07W (NORU), LOCATED APPROXIMATELY 122 NM
      SOUTH OF IWO TO, HAS TRACKED WESTWARD AT 05 KNOTS OVER THE PAST SIX
      HOURS. STY 07W HAS RAPIDLY INTENSIFIED OVER THE PAST 24 HOURS
      INCREASING 80 KNOTS FROM 60 KNOTS TO THE CURRENT INTENSITY OF 140
      KNOTS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES
      THAT THE SYSTEM IS ANNULAR WITH A SYMMETRIC RING OF DEEP CONVECTION
      SURROUNDING A 15-NM CLEAR EYE AND FAIRLY UNIFORM CLOUD TOP
      TEMPERATURES. THE INITIAL INTENSITY IS ASSESSED AT 140 KNOTS BASED
      ON A DVORAK INTENSITY ESTIMATE OF T7.0 (140 KNOTS) FROM PGTW.
      ENVIRONMENTAL CONDITIONS ARE FAVORABLE WITH RADIAL OUTFLOW, ENHANCED
      BY A POLEWARD OUTFLOW CHANNEL INTO THE TUTT AND SUBTROPICAL JET TO
      THE EAST-NORTHEAST. ADDITIONALLY, SST VALUES HAVE INCREASED TO 29 TO
      30C AS THE SYSTEM TRACKED SOUTH OVER THE PREVIOUS DAY. STY 07W IS
      DRIFTING WESTWARD WITHIN A COMPETING STEERING ENVIRONMENT.
      3. FORECAST REASONING.
      A. NO CHANGE TO THE FORECAST TRACK PHILOSOPHY SINCE THE PREVIOUS
      PROGNOSTIC REASONING MESSAGE. HOWEVER, FORECAST INTENSITY VALUES
      HAVE INCREASED SIGNIFICANTLY.
      B. STY 07W IS FORECAST TO TURN ONTO A NORTHWARD TRACK AFTER TAU
      24 AS THE EASTERN SUBTROPICAL RIDGE (STR) STRENGTHENS AND THE
      WESTERN STR WEAKENS OVER WESTERN JAPAN. THE EXACT TRACK IS DIFFICULT
      TO ASCERTAIN WITH AVAILABLE DYNAMIC MODEL TRACKERS INDICATING A WIDE
      SPREAD IN SOLUTIONS AT TAU 72, APPROXIMATELY 300NM. NAVGEM IS THE
      WESTERN MOST OUTLIER AND SHOWS A WEST-NORTHWESTWARD TRACK INTO THE
      RIDGE SO IS CONSIDERED SUSPECT. THE BULK OF THE NUMERICAL GUIDANCE
      IS ACTUALLY IN BETTER AGREEMENT WITH A 100NM SPREAD–GFS HAS TRENDED
      WESTWARD AND NOW SHOWS A MORE NORTHWARD TRACK VICE NORTHEASTWARD.
      THE INTENSITY FORECAST REMAINS UNCERTAIN IN LIGHT OF THE RECENT
      UNEXPECTED AND POORLY FORECAST RAPID INTENSIFICATION PHASE BUT, IN
      GENERAL, CONDITIONS APPEAR CONDUCIVE FOR INTENSITIES TO REMAIN
      GREATER THAN 120 KNOTS THROUGH TAU 72. THROUGH TAU 72, CONFIDENCE
      HAS INCREASED BUT REMAINS LOW DUE TO POOR TO FAIR AGREEMENT AMONGST
      THE MODEL VORTEX TRACKERS.
      C. AFTER TAU 72, UNCERTAINTY INCREASES WITH A 450NM SPREAD IN
      SOLUTIONS. IN GENERAL, THE MODELS HAVE BEGUN TO INDICATE DIFFERING
      DEGREES OF A WESTWARD TO NORTHWESTWARD TURN LATE IN THE FORECAST
      PERIOD. ANALYSIS OF MODEL FIELDS INDICATE THE STR OVER JAPAN WILL RE-
      BUILD IN RESPONSE TO A BUILDING MIDLATITUDE SHORTWAVE RIDGE.
      FORECASTS BEYOND TAU 120 ARE HIGHLY UNCERTAIN AND DEEMED TENUOUS AT
      THIS TIME. STY 07W IS FORECAST TO WEAKEN AFTER TAU 72 AS IT BEGINS
      TO TRACK OVER COOLER SST 26 TO 28C. OVERALL, THERE IS LOW CONFIDENCE
      IN THE JTWC FORECAST TRACK, HOWEVER, INTERNATIONAL FORECASTS HAVE
      REMAINED CONSISTENT AND IN CLOSE AGREEMENT WITH THE JTWC FORECAST
      DESPITE THE COMPLEX STEERING ENVIRONMENT.//
      NNNN

      Like

  1. Not quite sure what to make of Noru at the moment. Cloud tops have warmed dramatically, more than would be expected with diurnal cycles alone. Recent microwave imagery doesn’t appear to indicate that there are any system flaws. Recent IR images actually look a little bit like subtle eyewall replacement is taking place, boring the eye out to an even larger diameter. Confusing mixed signals for sure.

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s